Advanced engine control

ABSTRACT

A method for controlling an internal combustion engine to maintain a predetermined combustion efficiency, comprises: measuring the concentration of CO in an engine-out emission; determining whether the measured concentration of CO is above or below a target CO concentration for the engine under the operating conditions in use; and if the concentration of CO is above the target CO concentration, adjusting one or more engine and/or combustion parameters so as to reduce the concentration of CO in the engine-out emission, and if the concentration of CO is below the target CO concentration, adjusting one or more engine and/or combustion parameters so as to increase the concentration of CO in the engine-out emission; wherein the target CO concentration is determined on the basis of a correlation with indicators of combustion efficiency under known engine operating conditions. Exhaust gas recirculation (EGR) is one such engine and/or combustion parameter. A method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range is also described. The methods may be implemented by an engine control unit.

FIELD OF THE INVENTION

This invention relates to methods for controlling an internal combustion engine to control combustion stability and/or reduce engine-out nitrogen oxides (NOx) emissions. In particular, the invention relates to methods for using the measurement of carbon monoxide (CO) in the engine-out emissions to control combustion stability and/or indicate the level of NOx in the engine out emissions.

BACKGROUND OF THE INVENTION

Current emissions legislation in, for example, the US and Europe, set strict limits on the acceptable levels of polluting gases that are tolerated in the exhaust gas emissions of compression ignition engines; and engines using lean burn (oxygen rich) combustion, including diesel and gasoline direct injection engines. However, future legislation is set to pose a far more serious challenge to the manufacturers of such engines.

To overcome the challenge of reducing exhaust gas emissions, intense research and development is necessary in the areas of combustion, after-treatment or both.

The typical approach taken to improving the combustion process and reducing the production of undesirable engine gas emissions is to utilise some form of “advanced combustion”. Advanced combustion can be considered to be an umbrella term that encompasses any type of combustion process which aims to minimise the traditional flame burn of a combustion engine and replace it with simultaneous ignition of well mixed air and fuel. Examples of advanced combustion processes include Homogeneous Charge Compression Ignition (HCCI), Low-Temperature Combustion (LTC) and Pre-mixed Charge Compression Ignition (PCCI). Regardless of the name given to the process, all forms of advanced combustion generally attempt to achieve very low levels of NOx and soot emissions through improved fuel-air mixing and reduced combustion temperatures.

Advanced combustion typically involves one or more of the following aspects: fuel injection much advanced of Top Dead Centre (TDC); multiple fuel injections; high amounts of Exhaust Gas Recirculation (EGR); and high injection pressures.

It is generally accepted that some form of advanced combustion is necessary as a practical and cost effective means for internal combustion engines to meet present and certainly future emissions regulations. However, it has been found that during an advanced combustion process, even small errors (or changes) in air charge or combustion chamber temperature can lead to dramatic changes in engine emissions, combustion stability and engine noise.

Moreover, it should be appreciated that the control of advanced combustion is not merely a matter of air charge and engine controls, but also of fuel properties. The most significant of these fuels properties being cetane number, which is a measure of a fuel's ignitability. For instance, it has been shown that changes in cetane number can cause significant variations in both combustion and engine NOx emissions. In short, all of the available advanced combustion processes face a number of challenges, in terms of: combustion controls, system to system dispersion, combustion stability, variation in fuel properties, and estimating/modelling engine-out emissions.

Therefore, several systems under development seek to monitor engine performance during advanced combustion, and ultimately to reduce exhaust gas emissions. For example, engine control processes may utilise NOx sensors, Air-Fuel Ratio (AFR) sensors, ammonia sensors, and/or cylinder pressure sensors.

NOx sensors provide the most direct means of monitoring the engine-out NOx emissions from an internal combustion engine. Once the level of NOx has been determined, a feedback signal may be used to control EGR, in an attempt to adjust the amount of NOx produced. However, this system has several drawbacks. For instance, the concentration of NOx is typically very low during advanced combustion (e.g. approximately 4 ppm), so any NOx sensor must be extremely sensitive and accurate in order to provide a useful measurement. Even then, it is difficult to accurately control EGR, for example, to within 5%, so that even if a feedback mechanism is used to subsequently adjust EGR, any changes in NOx may be too crude to produce the desired result. Hence, despite the requirement for extremely highly sensitive measuring equipment, such a method, as implemented, lacks resolution. To compound the situation, it has also been found that NOx levels are not a good indicator of combustion stability, which means that NOx levels could be adjusted to the detriment of combustion stability and, thus, at the expense of hydrocarbon emissions, soot and fuel economy.

AFR sensors provide a fairly useful system for calculating the amount of EGR. Having determined the approximate level of EGR, the amount of NOx in the engine-out emissions can be estimated. If deemed necessary, a feedback signal can then be used to adjust the AFR through changes in both EGR and fuel injection quantity, so as ultimately to adjust the (estimated) NOx levels. A particular problem with this system, however, is that NOx is not measured directly and, therefore, the calculated values of NOx may be inaccurate. Also, as noted above, there is no direct link between NOx emissions and combustion stability.

In-cylinder pressure based controls can be used to indicate whether or not to remove EGR. For example, when the measured values of in-cylinder pressure become too variable (despite that NOx levels may be low), it is a good indication that EGR is too high and should be reduced. However, there are numerous disadvantages associated with reliance on in-cylinder pressure control. Primarily, such a system relies intensively on micro-processor power, and also the sensors used are not yet robust enough for reliability. Further disadvantages are that: there is no direct link between cylinder in-pressure and NOx levels; such a system may require multiple cylinder sensors (e.g. one in each cylinder) even to attempt to achieve a desirable effect; and a very rapid response from the sensors are needed to provide any meaningful data.

Hydrocarbon (HC) sensors can provide a means for determining the efficiency of the combustion process. In this regard, the higher the concentration of HCs in the engine-out emissions, the more unburned fuel is in those emissions. The level of HCs can then be used as a feedback signal to alter fuel injection quantity, injection timing and EGR, so as to improve the efficiency of combustion. However, although there is a correlation between HC and NOx concentration in the engine-out emission, this link can be broken in the event of misfire or wall wetting. In addition, HC sensors are sensitive to hydrogen, which means that in some circumstances measurements of HC concentration can be inaccurate.

Ammonia (NH₃) sensors are useful within a feedforward system for controlling Selective Catalytic Reduction (SCR), i.e. using a catalytic converter. For example, the amount of NH₃ measured in the engine-out emissions can be used to determine the amount of urea that must be injected into the catalytic converter in order to remove NOx in the engine-out emission by catalytic reduction. The amount of ammonia emitted should preferably be negligible. Therefore, is ammonia is detected in the engine-out emission, then too much urea was added during the SCR process. However, NH₃ levels cannot be directly linked to levels of engine-out NOx. Thus, although an NH₃ sensor may serve a useful purpose, e.g. as an indicator for optimising after-treatment of the engine-out emissions, it cannot be used as a means for controlling the combustion process itself.

Thus, there are significant problems associated with each of the known systems for monitoring engine-out emissions, and accordingly, none provides a completely satisfactory solution to the problem of reducing undesirable gas emissions, and particularly NOx emissions, in the exhaust gas of an internal combustion engine.

In recognition of the above shortfalls in the prior art, as already noted, some form of after-treatment is typically necessary to reduce exhaust gas emissions to the level required by emissions regulations.

Typical the after-treatment is provided by one or more device, such as a catalytic converter and/or a particulate filter. A catalytic converter may, for example, be designed to reduce NOx gases into O₂ and N₂. Alternatively, selective catalytic reduction, for instance, using urea injection, may be used to concert NOx gases into N₂ and water.

However, after-treatments, such as catalytic converters, can be economically undesirable, because of the costs of manufacture, the need to regularly monitor and replace old systems, and the running costs associated with the consumables, such as fuel, which are required for their operation.

Therefore, especially in view of the more stringent emissions criteria that will soon enter into force, there is a need for a system for accurately determining, and if necessary reducing, the levels of pollutants in engine-out emissions from internal combustion engines. More specifically, there is a need for a method for accurately determining the level of NOx in such engine-out emissions, and/or for adjusting an engine management system (which may include control over the combustion process, after-treatments, or both), so as to reduce the amount of NOx that is released into the atmosphere from the exhaust gas emissions.

One way in which levels of NOx (and other undesirable gases) in exhaust gas emissions may be controlled is by the control of combustion stability. However, the presently available sensors and systems do not provide a convenient mechanism by which combustion stability can be controlled.

Accordingly, there is an additional need for a system and method for controlling engine combustion, in particular, combustion stability, such that it is not necessary to additionally employ after-treatments, or such that any after-treatments can be used at a reduced level compared to current systems.

It would also be an advantage to have a method that achieves some of the above aims without significantly adding to the cost of manufacturing or running an internal combustion engine within the exhaust gas emissions regulations.

Present means of controlling combustion, and particularly means of controlling advanced combustion processes in a motor vehicle are based on monitoring various engine/combustion parameters, such as NOx production, and using an engine control unit to make adjustments to engine parameters to push the engine towards a set of conditions under which it is perceived that NOx production during the combustion process will be minimised. However, a difficulty with adjusting such a sensitive process as advanced combustion, especially when it is pushed in the direction of minimal NOx production levels (under which conditions combustion can become unstable), is that it is necessary to have the ability to: (i) rapidly monitor the combustion process in real time; (ii) accurately assess the combustion process; and (iii) sensitively adjust the combustion process, thereby to reduce NOx levels. Measuring NOx concentration in engine-out emissions is one method used in the art because, in theory, it provides a direct measurement of the concentration of NOx in the engine-out emissions. However, for the reasons discussed herein, the measurement of engine-out NOx emissions can be technically challenging, lacking in resolution, and the necessary equipment can be either impractical or expensive. In addition, it has been found that NOx emissions do not appear to link well to combustion stability. Therefore, a direct focus on NOx emissions can be at the detriment of combustion stability. Measuring CO levels in engine-out emissions, by comparison, is relatively straightforward, accurate and, as described herein, provides a link to both combustion stability and engine-out levels of NOx.

Thus, this invention aims to overcome or alleviate some of the problems associated with the prior art.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a method for controlling an internal combustion engine to maintain a predetermined combustion efficiency, the method comprising: measuring the concentration of CO in an engine-out emission; determining whether the measured concentration of CO is above or below a target CO concentration for the engine under the operating conditions in use; and if the concentration of CO is above the target CO concentration, adjusting one or more engine and/or combustion parameters so as to reduce the concentration of CO in the engine-out emission, and if the concentration of CO is below the target CO concentration, adjusting one or more engine and/or combustion parameters so as to increase the concentration of CO in the engine-out emission; wherein the target CO concentration is determined on the basis of a correlation with indicators of combustion efficiency under known engine operating conditions.

Preferably the one or more engine and/or combustion parameters is EGR, which, in accordance with the data provided herein, may be adjusted to change combustion conditions in a predictable manner.

Typically, the known engine operating conditions include engine speed and load, which can affect combustion parameters and hence, engine-out emissions. Conveniently, the predetermined combustion efficiency is selected during engine development and/or testing. In this way an engine control unit (ECU), for example, may be programmed to control engine and/or combustion parameters to achieve/maintain the desired level of combustion stability, and/or fuel economy and/or NOx emission level.

In one embodiment, the target CO concentration is determined on the basis of a correlation with one or more engine and/or combustion parameters which are indicators of combustion efficiency; wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and the one or more engine and/or combustion parameters. In preferred embodiments the means of data comparison is a look-up table, calibration curve or equation (or similar means) that has been obtained for the engine under known engine operating conditions, for example, conditions of engine speed and load. Most preferably, a plurality (or series) of look-up tables, calibration curves or equations are used so that a suitable data correlation can be conducted for any engine operating conditions in use.

In any or all aspects of the invention, there are some circumstances wherein the one or more known engine conditions that were used to obtain the one or more means of data comparison may not exactly correspond to the engine operating conditions (such as speed and load) when the method is in use. In such circumstances the correlation between engine-out CO concentration and indicators of combustion efficiency obtained under the known engine conditions may be considered to be an approximation of the correlation that exists under the engine operating conditions in use. When there is more than one correlation, i.e. where there is a plurality of means of data comparison, then the correlation obtained under the known operating conditions that are closest to the engine operating conditions in use will preferably provide the appropriate correlation for use.

Alternatively, or in addition, the correlation with indicators of combustion efficiency under known engine operating conditions may be based on an estimated correlation for the operating conditions in use. In this case, the estimated correlation may be obtained by adjusting the correlation obtained under the known engine operating conditions by means of inter alia an appropriate regression analysis, equation or algorithm. In other words, the trend in the correlation between engine-out CO concentration and the indicator of combustion efficiency (such as engine-out CO concentration) can be used to anticipate (estimate) the appropriate correlation under the conditions of use, based on the available correlation under the known operating conditions. Again, where there is more than one means of data comparison (obtained under different engine operating conditions), then preferably at least the correlation obtained under the conditions most similar to those in use are used as a basis for the estimation. As already noted, such a system may apply to any of the aspects of the invention mentioned herein.

In a preferred embodiment, the target CO concentration is predetermined on the basis of a correlation between CO concentration and hydrocarbon (HC) concentration under known engine operating conditions, the target CO concentration being that which corresponds to a predetermined HC concentration at which a predetermined acceptable level of combustion stability is achieved. As described below, HC concentration is a good indicator of combustion efficiency and stability, which have predictable effects on fuel economy. The preferred fuel economy and/or combustion stability may be selected or determined during engine testing and development, for example; and may be selected in accordance with any required engine specifications or performance characteristics considered appropriate. Thus, the target CO concentration may, for example, be the concentration of CO in the engine-out emission that corresponds to an engine-out HC concentration of 350 to 450 ppm. In a preferred embodiment the target CO concentration is that which corresponds to an HC concentration of approximately 400 ppm.

In a second aspect there is provided a method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is within the predetermined concentration range; and where the calculated concentration of NOx is outside of the predetermined concentration range, adjusting one or more engine and/or combustion parameters so as to achieve an engine-out emission level of NOx within the predetermined concentration range.

The predetermined concentration range of NOx may be any appropriate range, for example, a range that is specified by an engine or motor vehicle manufacturer, an end user, or a legal regulation or guideline. Preferably, the predetermined concentration range is sufficiently low that by use of exhaust after-treatments, the engine-out emission level of NOx may be reduced to below national or international NOx emissions criteria. More preferably, the predetermined concentration range is no greater than one or more national or international NOx emissions criteria, so that after-treatments are not required.

Thus, in preferred embodiments the predetermined concentration range of NOx may be in the range: 0 to 0.20 g/mile for Tier II Bin 8; 0 to 0.07 g/mile for Tier II Bin 5; 0 to 0.25 g/km for Euro IV; 0 to approximately 1.25 g/hp*hr for 2007 and later US On-Highway HD Engines; and 0 to 0.20 g/hp*hr for 2010 and later US On-Highway HD Engines. However, these preferred embodiments are in no way intended to restrict the scope of the invention, which is also suitable for vehicles required to meet more stringent emissions regulations such as Tier II Bin 3, or Euro V, which will be known to the skilled person in the art.

In preferred embodiments the one or more engine and/or combustion parameters is EGR and, for example, EGR is increased when the calculated concentration of NOx is higher than the predetermined concentration range.

In the alternative, or in addition, the one or more engine and/or combustion parameters is fuel injection timing. In a further embodiment, the one or more engine and/or combustion parameters is injected fuel quantity.

In preferred embodiments of the invention, an engine control unit is used to: calculate the concentration of NOx on the basis of the measured concentration of CO; and adjust one or more engine and/or combustion parameters using a feedback signal.

Preferably in this aspect of the invention the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters. As in all aspect and methods of the invention, the means of data comparison may be one or more look-up tables, calibration curves or equations.

In some embodiments this method of the invention may further comprise the steps of: determining whether the calculated concentration of NOx is below a predetermined concentration level; and where the calculated concentration of NOx is above the predetermined concentration level, operating one or more after-treatments to reduce the concentration of NOx in said engine-out emission to a level no greater than said predetermined concentration level. Thus, the method of the invention may, for example, be used to control SCR (e.g. to avoid ammonia slip), or to model NOx storage in an NOx adsorber.

Accordingly, in a third aspect of the invention there is provided a method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is above a predetermined concentration level; and where the calculated concentration of NOx is above the predetermined concentration level, operating one or more after-treatments to reduce the concentration of NOx in said engine-out emission to a level no greater than said predetermined concentration level.

In one embodiment of this aspect of the invention an engine control unit is used to: calculate the difference between the calculated concentration of NOx and the predetermined concentration level; and adjust the one or more after-treatments to reduce the concentration of NOx in said engine-out emission to a level no greater than the predetermined concentration level, and wherein the adjustment to the one or more after-treatments is dependent on the difference between the calculated concentration of NOx and the predetermined concentration level.

Any suitable after treatment may be used. Preferably, the one or more after-treatments is selected from the group consisting of: a diesel particulate filter, a catalytic converter, selective catalytic reduction and an NOx trap or NOx adsorber.

As in other aspects of the invention, the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters; and the means of data comparison is a look-up table, calibration curve or equation. Preferably a plurality or series of such look-up tables, calibration curves or equations is used. Alternatively, the means of data comparison may be a computer model or models.

Accordingly, a fourth aspect of the invention provides a method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is within the predetermined concentration range; and where the calculated concentration of NOx is outside of the predetermined concentration range, adjusting one or more engine and/or combustion parameters so as to achieve an engine-out emission level of NOx within the predetermined concentration range; and operating one or more after-treatments to reduce the concentration of NOx in said engine-out emission.

In a preferred embodiments, the method of this aspect of the invention further comprises the steps of: determining whether the calculated concentration of NOx is above a predetermined concentration level; and where the calculated concentration of NOx is above the predetermined concentration level, operating the one or more after-treatments to reduce the concentration of NOx in the engine-out emission to a level no greater than the predetermined concentration level.

In a fifth aspect there is provided a method for calculating the concentration of NOx in an engine-out emission from an engine, the method comprising: measuring the concentration of CO in said engine-out emission; and calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO under the engine operating conditions in use; wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters.

Similarly, in a sixth aspect of the invention, there is provided a method for calculating the engine operating Air-to-Fuel Ratio (AFR) in an engine, the method comprising: measuring the concentration of CO in the engine-out emission from said engine; and calculating the AFR of said engine based on a correlation with the measured concentration of CO under the operating conditions in use; wherein said correlation is by a means of data comparison, said data relating to measured AFR levels and concentrations of CO in engine-out emissions under known engine and/or combustion parameters.

Typically, in either or both of the fifth and sixth aspects of the invention (as in other aspect of the invention), the means of data comparison is by means of one or more look-up tables, calibration curves, equations or computer models; and where necessary, the correlation may be based on an approximation of the correlation under the engine operating conditions in use, or an estimation of that correlation. Preferably, the correlation is performed by an engine control unit.

In a seventh aspect there is provides the use of a CO sensor in a system for controlling or adjusting one or more engine and/or combustion parameters in an internal combustion engine, wherein the CO sensor is arranged to measure the concentration of CO in an engine-out emission, and the system includes a means for correlating the measured concentration of CO with one or more of: an indicator of combustion efficiency under the operating conditions in use; the concentration of NOx in the engine-out emission under the operating conditions in use; and/or the level of AFR in the engine under the operating conditions in use.

In one embodiment, the system controls or adjusts one or more engine and/or combustion parameters in order to bring the combustion efficiency to within a predetermined concentration range. In another embodiment, the system controls or adjusts one or more engine and/or combustion parameters in order to bring the calculated concentration of NOx in the engine-out emission to within a predetermined concentration range. In a further embodiment the system controls or adjusts one or more engine and/or combustion parameters in order to bring the calculated level of AFR in the engine to within a predetermined range. Preferably, in one aspect of the invention, the indicator of combustion efficiency is hydrocarbon (HC) concentration in the engine-out emission.

The invention also provides an engine control unit for a motor vehicle that, in use, performs according to the method of any aspect or embodiment of the invention. The invention further provides a motor vehicle comprising such an engine control unit.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 demonstrates the relationship between: the coefficient of variance (COV) of “mean effective pressure” (MEP) within a combustion cylinder (left-hand axis, filled triangles); and engine-out HC emissions (right-hand axis, filled diamonds), as a function of NOx emissions for a North American V8 Diesel engine under steady state operating conditions of 1200 rpm, 59 Nm;

FIG. 2 demonstrates the relationship between engine-out emissions of NOx and the amount of EGR used during advanced combustion in a V8 diesel engine operating at an engine speed and load of 1600 rpm, 130 Nm (filled diamonds, dotted line); and 1200 rpm, 59 Nm (filled square, solid line);

FIG. 3 demonstrates the variation in NOx emissions from an advanced combustion diesel engine in relation to fuel injection timing for three different fuels, with cetane numbers (CN) of: 25 (open circles); 40 (open triangles); 53 (filled circles);

FIG. 4 demonstrates the relationship between engine-out emissions of CO and engine-out emissions of NOx during advanced combustion under three different engine speeds and loads: 1200 rpm, 59 Nm (filled squares, solid line); 1600 rpm, 50 Nm (filled triangles, dashed line); and 1600 rpm, 130 Nm (filled diamonds, dotted line);

FIG. 5 demonstrates the relationship between engine-out emissions of CO and engine-out emissions of HCs during advanced combustion under three different engine speeds and loads: 1200 rpm, 59 Nm (filled squares, solid line); 1600 rpm, 50 Nm (filled triangles, dashed line); and 1600 rpm, 130 Nm (filled diamonds, dotted line);

FIG. 6 demonstrates the relationship between engine-out emissions of CO and air to fuel ration (AFR) for a North American V8 Diesel engine capable of Tier II Bin 8 emissions (in a 6000 lb vehicle), under three different engine speeds and loads: 1200 rpm, 59 Nm (filled squares, left-hand vertical axis); 1600 rpm, 173 Nm (filled diamonds, left-hand vertical axis); and 1600 rpm, 297 Nm (filled triangles, right-hand vertical axis);

FIG. 7 is a schematic representation of a feedback algorithm for adjusting EGR on the basis of the difference between a target engine-out CO concentration and a measured engine-out CO concentration;

FIG. 8 is a second schematic representation of a feedback algorithm for adjusting fuel injection timing on the basis of the difference between a target engine-out CO concentration and a measured engine-out CO concentration in relation to a maximum allowable engine-out CO concentration;

FIG. 9 is a schematic representation of an algorithm for calculating engine-out NOx levels on the basis of the difference between a target engine-out CO concentration (correlating to a target NOx level) and a measured engine-out CO concentration (correlating to a calculated NOx level);

FIG. 10 is a schematic representation of an algorithm for calculating AFR on the basis of engine-out CO concentration.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “engine-out emission(s)” refers to the emissions, which may be in the form of gas, liquid or particulate matter, from an engine combustion chamber itself. Thus, an engine-out emission has not undergone any so-called “after-treatments”, which are positioned downstream of the combustion chamber. By way of contrast the term “exhaust gas emission(s)” is used to refer to the emissions, which again may be gas, liquid or particulate, that are released from the exhaust pipe into the atmosphere. Thus, an “exhaust gas emission” may have undergone one or more after-treatments (e.g. in a catalytic converter) to remove undesirable molecules contained in an engine-out emission. Where no after-treatments are used, the engine-out emissions can be assumed to be the same as the exhaust gas emissions.

Advanced Combustion

As noted above, there are a number of internal combustion processes that aim to reduce engine-out and exhaust gas emissions, and can be classified as operating a form of “advanced combustion”.

Homogeneous Charge Compression Ignition

One form of advanced combustion is Homogeneous Charge Compression Ignition (HCCI), in which well mixed fuel and air (although another oxidiser could replace the air) are compressed to a pressure at which they auto-ignite. HCCI combines aspects of the internal combustion processes of both gasoline engines, i.e. homogeneous charge spark ignition, and diesel engines, i.e. stratified charge compression ignition. However, in contrast to the gasoline system, rather than using a spark to ignite the fuel-air mixture, compression is used to raise the temperature and concentration of the mixture until the entire fuel-air mixture reacts (simultaneously). In a typical diesel injection engine, compression is also used to increase temperature and gas concentration, but the combustion event begins at the interface between the injected fuel and the compressed combustion chamber gases.

Since in HCCI, ignition of the fuel-air mixture can occur in several places at the same time, the entire gas content of the combustion chamber can combust almost simultaneously. This fact, combined with the lack of a direct ignition event (e.g. from a spark at a predetermined time), makes the HCCI process inherently difficult to control. For example, if auto-ignition occurs too early, or with too much ferocity, there is the risk that the extremely high gas pressure inside the cylinder could destroy an engine. Therefore, to avoid this situation, it is typical for HCCI to be carried out with lean fuel mixtures.

The homogenous mixing of fuel and air before ignition and the lean fuel mixtures in the HCCI process achieve a number of advantages over alternative combustion processes, including excellent fuel economy and cleaner combustion, which results in low engine emissions. In particular, due to the lower combustion temperatures, HCCI engines are known to achieve very low levels of NOx emissions, which under some emissions regulations may obviate the need for catalytic converter after-treatments.

However, the low temperature combustion in HCCI has the disadvantage that CO and HC emissions are typically relatively high, so that appropriate after-treatments may be required to meet those respective emissions criteria.

Two of the main disadvantages of current HCCI processes are: (i) the difficulty in controlling the process (as already noted); and (ii) the limited power range available. These issues are discussed below.

In contrast to the tightly controlled ignition events of other popular combustion systems, such as gasoline engines (which ignite with a spark) and direct-injection diesel engines (which ignite when fuel in injected into compressed air); in HHCI, the homogeneous mixture of fuel and air is compressed, and combustion begins whenever the appropriate conditions of fuel-air pressure and temperature are reached. Hence, there is no well-defined combustion initiation event that can be directly controlled. Control of ignition in HCCI is, therefore, a major hurdle that needs to be overcome.

The concentration and/or temperature of the fuel and air mixture in an HCCI engine can be increased (i.e. to trigger combustion) in several different ways, for example, by using: (i) a high compression ratio; (ii) pre-heat induction gases; (iii) forced induction; and (iv) exhaust gas retention/recirculation.

These processes can be adjusted so that the appropriate ignition conditions occur at a desirable timing. However, these conditions only apply under specific operating parameters of, e.g. engine work, torque and fuel composition.

In order to dynamically control an HCCI engine, the engine management/control system must be capable of adjusting engine conditions, for example, to control combustion timing, in response to real-time signals (or data) obtained from the engine system. Thus, in response to an appropriate signal, the engine management system would beneficially adjust (and monitor) one or more of: the compression ratio, the inducted gas temperature, the inducted gas pressure, or the quantity of retained or re-inducted exhaust.

Variable Compression Ratio

There are various methods known to the person of skill in the art for adjusting the compression ratio in a combustion cylinder. By way of example, the “effective” compression ratio can be altered by changing the timing of closure of the fuel/air intake valve(s). Thus, by closing the intake valve(s) late in the combustion cycle, the effective compression ratio will be reduced, and by closing the intake valve(s) early, the effecting compression ratio will be increased. This may be known as variable valve actuation. Alternatively or in addition, the “geometric” compression ratio may be varied using a movable plunger at the top of the cylinder head, to relatively increase or decrease the volume of the combustion chamber. These methods are technically quite difficult to implement and can be expensive.

Variable Induction Temperature

This technique requires a means of rapidly changing the temperature of the intake fuel (charge), from one combustion cycle to the next. Such a system requires extremely fast thermal management, may be expensive, and has a limited operational range.

Variable Exhaust Gas Percentage

Exhaust gas can be very hot if retained from the preceding combustion cycle. In this case, the hot combustion will increase the temperature of the gases in the combustion cylinder, and will typically advance the combustion event. In contrast, if EGR is recirculated through the intake (as in conventional systems), the gases entering the combustion chamber are cooler and the fresh charge is diluted. As a consequence, ignition is delayed.

EGR is perhaps the most popular form of NOx reduction technique, since it is used in many engine types. Dilution and mixing of engine-in fuel and air with recirculated exhaust gas dilutes the fuel with some inert gas, thereby lowering the adiabatic flame temperature and (in diesel engines) reducing excess oxygen. Another benefit is that the exhaust gas increases the specific heat capacity of the mix, which lowers the peak combustion temperature, and reduces the generation of NOx.

Variable Valve Actuation

Variable valve actuation can be a complicated and expensive system, but advantageously, it can be used to control both the compression ratio and the exhaust gas percentage.

High Peak Pressures and Heat Release Rates

In HCCI, because the entire fuel/air mixture ignites and burns almost simultaneously, the process involves higher peak pressures within the combustion cylinder, and higher energy release rates, than in other combustion processes. Thus, to physically withstand these higher pressures, an HCCI engine must be structurally stronger, and consequently heavier. To reduce the need for heavier engines, it has been proposed to reduce the combustion rate. One such proposal is to mix two different fuel, having different combustabilities, such that combustion occurs over a longer period. However, it is then necessary to provide an entirely different engine structure to accommodate the different fuel supplies. In addition, the lower combustion pressure and rate leads to reduced engine power.

Power

In non-advanced combustion engines, power output is simply increased by increasing the amount of fuel (or fuel/air charge) that is injected in to the combustion chamber. In HCCI combustion, however, the problem of the high peak pressures (discussed above) is also relevant to maximum power output. Since the entire fuel/air mixture combusts almost simultaneously, to increase power by increasing the fuel to air ratio will result in higher peak pressures and higher heat release rates. In addition, some of the common processes used in HCCI for controlling combustion, such as exhaust gas retention, mean that the fuel and air mix is preheated, and accordingly less dense. Therefore, the amount of fuel contained in the combustion chamber is actually reduced. Finally, “knocking”, which is generally reduced or eliminated in HCCI engines, can become a problem at high fuel to air ratios.

For these reasons, increasing the power output from a HCCI engine is quite difficult.

One way of increasing power output can be to use different types or blends of fuels, as discussed above, by extending the combustion period. Alternatively, it may be possible to produce thermal gradients in the combustion cylinder, such that different regions combust at slightly different times. Another method is to run the engine in HCCI mode only for those periods during which the engine load is within the optimal conditions for HCCI, and to run it by the conventional spark or diesel ignition modes during other periods.

-   -   All of these issues and potential solutions can have serious         consequences in terms of engine-out emissions, and these must         also be addressed in order for advanced combustion engines to         remain within emissions regulations.

Premixed Charge Compression Ignition

Premixed charge compression ignition (PCCI) aims to address the problems of combustion control and specific power output in HCCI engines. PCCI can be considered to be a generalisation of HCCI, in which the fuel and air mixture may be partially stratified at the moment of ignition (some of these measures have already been discussed under “Homogeneous Charge Compression Ignition”, above).

Thus, PCCI engines can include direct injected engines with early injection, and controlled auto-ignition engines that use variable valve timing and high residual exhaust gas fraction to control combustion. In PCCI engines, the stratification of the fuel to air ratio (equivalence ratio) may be used for lengthening the burn duration, thereby allowing the engine to operate at higher specific power. Engine control can also be aided by direct injection of fuel into the combustion chamber, especially if the injected fuel is highly reactive, such as diesel fuel. However, as already noted, a significant disadvantage of fuel stratification is that high NOx and particulate matter emissions may result.

Fuel Injection Timing

As indicated above, fuel injection timing can be an important aspect of advanced combustion, both in terms of producing a relatively homogeneous fuel/air mixture to achieve the benefits of HCCI, but also in terms of controlling the combustion and power output.

Port Injection

The easiest way of creating a homogenous in-cylinder fuel/air mixture is by injecting fuel upstream of the intake valves, and inducting the mixture into the cylinder during the intake stroke, such that the turbulence generated during intake promotes mixing. This approach is the most common system used in HCCI engines. However, it has some disadvantages. First, because fuel is necessarily injected early, there is no option for adjusting injection timing in order to control the start of combustion. Secondly, it can result in high HC and CO emissions, as well as in increased fuel consumption and oil dilution. These problems are particularly relevant for less volatile diesel fuel.

Early In-Cylinder Injection

Along similar lines to the above method, early in-cylinder injection (i.e. well in advance of TDC), of all or part of the fuel, can help promote a homogeneous fuel/air mixture. As above, this system does not allow effective control of the combustion process through varying the fuel introduction time. Another concern is the amount of fuel wall impingement, particularly when heavy fuels such as diesel are injected into the low-density environment of the combustion chamber. To address this concern, low-penetration fuel injectors are being developed. Despite its drawbacks, early in-cylinder injection is gaining popularity in HCCI engines.

Late In-Cylinder Injection

As the name suggests, in this system, fuel is injected directly into the combustion chamber near or after TDC. However, the system is able to delay the ignition event by: (i) using large amounts of cooled EGR; (ii) reducing the engine compression ratio; and (iii) generating vigorous swirl. In this way, combustion can be delayed until well after the end of injection. Advantageously, low NOx emissions are achieved despite the lack of mixture homogeneity within the cylinder. Moreover, in the case of diesel fuel, the problems associated with fuel wall impingement (e.g. of early in-cylinder injection) are reduced or avoided, and some control over the combustion timing is enabled. However, at high engine loads it may not always be possible to delay ignition until after the end of fuel injection.

Thus, a preferred mode of operating a diesel HCCI engine in accordance with the invention involves the use of late in-cylinder injection of fuel.

Exhaust Gas Emissions

Although HCCI can be extremely effective in reducing engine-out emissions, a combustion system has not yet been developed that will be able to meet future exhaust gas emissions regulations over a broad engine operating range. The engine-out emissions relevant in HCCI are briefly discussed below.

Nitrogen Oxides (NOx)

The greatest general benefit of HCCI is the reductions that can be achieved in engine-out NOx emissions. For example, typical NOx emissions from HCCI may be over 90% lower than in conventional Diesel combustion engines. As already noted, this benefit is achieved through the lower combustion temperatures reached in an HCCI engine. Since combustion temperature is proportional to the fuel/air ratio, as engine load increases, NOx emissions typically also increase (Bergman & Golovitchev, 2005). Thus, HCCI combustion is practical only at low engine loads and low fuel/air ratios. NOx emission from HCCI combustion of Diesel fuel has been modelled (Dodge et al., 1998; Dickey et al., 1999), and it was found that the beneficial reduction in NOx emissions from HCCI at low engine loads, in comparison to conventional direct-injection diesel combustion engines, was lost at high engine loads. In a similar study on the effect of combustion phasing, it was also found that at high engine loads, premature ignition resulted in a large increase in NOx emission levels; while under low and medium loads, the NOx levels remained acceptably low.

Particulate Matter (PM)

HCCI combustion has also been reported to produce low levels of smoke and PM emissions (e.g. Suzuki et al., 1997; Mase et al., 1998). In this regard, it is likely that the homogeneous charge removes fuel-rich regions and encourages even combustion. However, the processes operating within HCCI to reduce NOx emissions, such as high EGR, can lead to increased PM formation, due to lower combustion temperatures and lower oxygen availability. Also, under some operating conditions, for example, with late in-cylinder injection, deposition of liquid fuel on the combustion chamber wall may cause localised fuel-rich regions, which promote soot formation. In general, PM emissions tend to increase in HCCI combustion as the fuel/air ratio is increased.

Hydrocarbons (HC) and Carbon Monoxide (CO)

As already mentioned, the lean fuel mixtures (i.e. low fuel to air ratio) used in HCCI engines means that the combustion process typically operates at lower peak temperatures than in other engines. These lower peak temperatures are beneficial in reducing the amount of engine-out NOx gases produced, which is a primary concern in regard to emissions regulations. However, in contrast to the generally advantageous NOx emissions in HCCI combustion, HC and CO emissions are typically higher than in conventional direct-injection diesel engines (Suzuki et al., 1997).

Probably, the most significant causal factor for the high HC and CO emissions is the low combustion temperature, which leads to lower oxidation rates within the combustion chamber. As discussed, this is an inevitable consequence of the lean fuel mixtures and high levels of EGR that are necessary to perform HCCI with low NOx emission. Again, liquid fuel deposition on combustion chamber walls (a particular problem with diesel fuel) may also result in increased HC emission levels (Stanglmaier et al., 1999).

In addition, in combustion models it has been found that CO concentration increases with the fuel/air ratio (Bergman et al., 2005), probably because the oxidation of CO to CO₂ is inhibited by lack of oxygen at high equivalence ratios. Meanwhile, at low fuel to air ratios, CO emissions have been found to increase at low temperatures.

CO emissions can, therefore, be considered to be a side effect of both low soot and NOx emissions. The present invention benefits from the higher levels of CO emissions in HCCI combustion, which is demonstrated by the data provided herein, to provide a means of not only monitoring engine performance, but also monitoring NOx emissions.

After-Treatments

As discussed, in order to meet emissions regulations in the EU and the US, diesel engines typically require one or more after-treatment device, to concert the engine-out emissions into exhaust gas emissions having acceptable levels of pollutants. The most common forms of after-treatments for diesel engines are a diesel particulate filter and a catalytic converter. In fact, both of these devices will typically be used, and may be positioned sequentially within the same housing, for example, within an expansion chamber.

Diesel Particulate Filter (DPF)

A DPF is designed to remove particulate matter (e.g. soot) from the engine-out emissions of a diesel engine. The efficiency of approved DPF devices tends to be in the region of 85-90%, such that, when fitted, no visible smoke should be emitted from an exhaust pipe.

By its nature a DPF requires maintenance, because otherwise the filtered soot (and other particles) that builds up on the inlet face of the filter can eventually lead to clogging of the filter. If too much PM is retained by the DPF, the pressure differential (back pressure) created across the filter can lead to engine damage. Therefore, it is necessary to either replace old DPFs or employ a means of regenerating the DPF to remove the PM from the filter.

There are several varieties of filter, which may be disposable, or in some cases can be regenerated.

Cordierite wall flow filters, which contain a ceramic material are the most common type of diesel filter. These filters are simple to use, relatively inexpensive and are highly efficient. It is possible to regenerate cordierite filters by burning off the captured soot particles. However, they have a relatively low melting point (approximately 1200° C.), which means that there is a danger of them melting during some regeneration procedures.

Silicon carbide wall flow filters are another popular filter type, which can be interchanged with cordierite wall flow filters in an engine exhaust. Although the silicon carbide cores have a higher melting point (1700° C.) than cordierite, they are thermally less stable and can be more expensive to use.

Metal fibre flow-through filters have cores of metal fibres, which are generally woven. These have the advantage that they can be readily heated for regeneration purposes by passing an electric current through the core. However, a disadvantage is that they are relatively expensive and not interchangeable with the above filters.

Disposable paper cores are not commonly used. However, they are used in certain circumstances where vehicles are used in-doors. The engine-out emissions are usually cooled through a water trap before contacting the paper.

Partial filters achieve between 50% and 85% PM filtration. The most common application of these filters is for retrofit; i.e. when a vehicle was not initially manufactured with a PM filter.

Regeneration of DPFs

DPFs may be regenerated by heating the filter to temperatures of approximately 600° C. or above, so as to burn off any PM. By adding a catalyst to the DPF, the regeneration temperature can be reduced, for example, to about 300° C. In catalysed DPFs regeneration can be aided by increased amounts of NO₂, while the process may be hindered by sulphur. Regeneration can be “passive”; or when the DPF is heated, for example, using a fuel burner, it is considered to be an “active” mechanism.

An active mechanism of regeneration may be triggered by an engine management system (computer) that responds to the load of PM on the DPF, for example, by measuring the back pressure and/or the temperature of the filter. Active DPF management may use one of a variety of systems to increase the exhaust temperature, such as: a fuel burner; engine management; a catalytic oxidiser; resistive heating coils; or microwave energy. All of these systems require fuel for operation. Therefore, it is uneconomical to run a regeneration cycle too frequently, but potentially damaging to the engine to allow too much build up of PM.

The methods and engine control systems of the present invention encompass active regeneration systems for DPF, when used.

Catalytic Converter (CC)

A catalytic converter (CC) can be used to reduce or eliminate levels of CO, HC and NOx in engine-out emissions. The conversion reactions that are catalysed by the CC are as follows:

oxidation of CO: 2CO+O₂→2CO₂  (i)

oxidation of HCs: CxHy+nO₂→xCO₂+mH₂O  (ii)

reduction of NOx: 2NOx→xO₂+N₂  (iii)

Under earlier emissions regulations it was possible to use a “two-way” CC for treating emissions from a diesel engine, because it was only necessary to reduce the exhaust emission of CO and HCs. However, under current emissions regulations (and probably also future legislation) it is preferable to employ a “three-way” CC, which is able to remove CO, HCs and NOx from the exhaust emissions. That said, typically, three-way CCs are not practical for use with a diesel engine. Therefore, it is more likely that two different types of catalyst will be employed concurrently, such as an oxidations catalyst and an NOx adsorber or SCR. Furthermore, it is possible that where an advanced combustion system is operated in such a way as to sufficiently reduce engine-out NOx emissions (for example, using the systems and methods of the invention), it may again be possible to use only a two-way CC. It is particularly advantageous to reduce the engine-out emissions of NOx (i.e. pre-after-treatment) in advanced combustion systems, because the lean fuel mixtures used in advanced combustion lead to high oxygen levels (e.g. up to 15%) in the engine-out emissions, which can greatly inhibit the ability of a CC after-treatment to reduce NOx to N₂.

The catalytic component of a CC is typically a precious metal, such as platinum. However, due to its expense and the possibility of unwanted side-reactions, palladium and/or rhodium may alternatively be used. Other materials such as cerium, iron, manganese and nickel could also be used. Further components of a CC are the core (or substrate), which is generally a ceramic or stainless steel honeycomb and is used to support the catalyst; and the washcoat, which is usually a mixture of silicon and aluminium, and is used to provide a rough surface (i.e. a large surface area for reaction).

An example of a CC that may be used is the Plasma-Assisted Catalytic Reduction process (PACR), which oxidises NO and HCs to NO₂ and partially oxidised HCs, respectively, which are then reduced over a catalyst to N₂, CO₂ and water.

In a diesel engine, it is common to use an “oxidation catalyst” to oxidise CO and HCs as already described above. An oxidation catalyst can remove 90% of these pollutants. However, levels of NOx cannot be reduced in such an oxidising environment.

Hence, to remove NOx from engine-out emissions, it is generally necessary to employ an alternative technology, such as an NOx trap or “selective catalytic reduction” (SCR).

Selective Catalytic Reduction (SCR)

In SCR a reductant, such as ammonia or urea, is added to the engine-out emission stream so that it is absorbed onto a catalyst in a CC. The reductant then reacts with any NOx in the engine-out emissions, converting it into N₂ and water. Thus, SCR requires both a reductant and a catalyst. An engine control unit is typically used to control the amount of reductant that is released into the engine-out emission stream. For the SCR process to be successful, special types of catalyst are required, for instance, catalysts based on vanadium, or having zeolites in the washcoat. In addition, the reaction process can be temperature dependent, so it can be necessary for the engine control unit to also regulate the temperature of the engine-out emissions that contact the CC. A disadvantage of the process is that the catalyst can require regular cleaning/maintenance, because the reductant can solidify onto the catalyst, particularly when the engine-out emissions are too cold, and especially if the reductant contains any impurities. In particular, the urea injection system in SCR can be difficult to design robustly for use in cold conditions.

The choice of reductant depends on various factors. Urea tends to be a preferred reductant in motor vehicles because it is less toxic than ammonia. However, the use of urea generates ammonia in the exhaust system. This can be a particular concern when unreacted reductants are released in the exhaust gas emissions (e.g. ammonia slip). The benefit of using ammonia, however, is that it is a more effective reductant of NOx. Different SCR equipment may be required, depending on the selection of reductant.

NOx Trap

A NOx trap provides a supplement (or an alternative) to the use of techniques, such as EGR and SCR, which are designed to reduce NOx in exhaust gas emissions.

An NOx trap (or NOx adsorber) requires a zeolite material, which can be included into a CC by applying it with a washcoat. As the name suggests, an NOx trap simply traps NO and NO₂ molecules, preventing them from being released in the exhaust gas emissions. Of course, once the trap is full, it must either be replaced or regenerated. Regeneration (“purging”) can be achieved, by injecting diesel (or another reactive agent), into the engine-out emissions up-stream of the NOx trap. The HCs in the reactant will react with the NOx to produce water and N₂.

Currently, NOx traps are relatively expensive.

In accordance with the methods of the present invention, a three-way CC may be employed to reduce/eliminate engine-out emissions of CO, HC and NOx. Preferably, a two-way CC is used where the engine-out emissions are of a composition that would inhibit the reduction of NOx gases in a three-way CC. In alternative embodiments, an NOx trap or SCR may be employed to reduce NOx emissions, in addition to or instead of either a three-way or an two-way CC. More preferably, no CC is used, for instance, in cases where the combustion process can be operated under conditions where the engine-out emissions of each of the undesirable pollutants (such as NOx) are reduced to an acceptable level in the engine-out emissions. Most preferably, no after-treatments are used when the levels of undesirable pollutants are sufficiently low in the engine-out emissions. In certain preferred embodiments, an engine control unit is used to control the release of reductant in SCR and/or the release of reactant for an NOx trap, according to the requirements of the after-treatment processes used.

Sensors

To regulate and/or reduce the exhaust gas emissions from an internal combustion engine, it is preferable to attempt to minimise (or eliminate) the production of undesirable emissions at the combustion stage, so that reliance on after-treatments can be reduced or even obviated. In this regard, although an internal combustion engine can be pre-programmed to operate according to parameters shown to reduce engine emissions, e.g. during engine testing; it is known that engine-out emissions can vary considerably during actual use. For example, according to the type of fuel used, the engine operating conditions (e.g. engine load), and the age of the engine (e.g. due to engine wear).

Therefore, the most effective means of maintaining low engine-out emissions would appear to involve a reactive mechanism, in which the emissions are regularly (or continuously) monitored, and an engine control unit is provided to feedback signals to change/control the engine operating conditions, according to the levels of pollutants detected. In this way, one or more engine parameters can be adjusted, in real time, to optimise the engine running conditions (i.e. attempt to minimise the production of undesirable engine-out emission gases), at that point in time.

A further advantage of monitoring the engine-out emissions is that an engine control unit can be provided to feedforward signals to change/control after-treatment processes, such that any after-treatments, when used, are operated under the optimal conditions in relation to the levels of the various engine-out emission gases at that point in time.

To employ an engine control unit in this manner, it is, of course, necessary to know the levels (concentration) of one or more of the gases of interest in the engine-out emissions. It is, therefore, convenient to provide one or more sensors in the exhaust system downstream of the combustion cylinder, but upstream of any after-treatments, if used. Such a sensor can detect or measure gas, liquid or particle concentration in the engine-out emissions, before those emissions are released into the atmosphere (as exhaust gas emissions), and before they have been exposed to any after-treatments.

In addition, one or more of the same or different sensors may be provided in the exhaust system downstream of any after-treatments that may be used, so that the amount of any particular component of the exhaust gas emission can be determined.

As has been mentioned above, there are a number of sensors that may advantageously be used to detect or measure the concentrations of one or more components of interest in the engine-out emissions from an internal combustion engine (and particularly from an advanced combustion engine). These sensors include: NOx sensors, Oxygen sensors (in conjunction with AFR meters), HC sensors, and ammonia sensors; some of which are reviewed by Sheikh et al., “Ceramic sensors for industrial applications” (http://www.mse.eng.ohio-state.edu/˜akbar/paper.htm) In addition, some sensors, such as in-cylinder pressure sensors and oxygen sensors may be used to indirectly estimate the concentration of relevant engine-out emission gases, e.g. NOx.

Oxides of Nitrogen (NOx) Sensors

NOx sensors provide the only direct means of monitoring the engine-out NOx emissions from an internal combustion engine. The term “NOx” relates to all of the oxides of nitrogen, including but not limited to: NO (the most common nitrogen oxide in engine-out emissions), NO₂, and N₂O.

Most of the NOx sensors that are currently available are constructed from metal oxides, such as yttria-stabilized zirconia (YSZ), which are compacted into a dense ceramic material that can conduct oxygen ions (O2-). High temperature electrodes made from relatively inert metals (such as platinum, gold, or palladium), or certain metal oxides, are placed on the ceramic material and an electrical signal (for example, resulting from a change in current or voltage within the sensor) is measured as a function of NOx concentration.

Sensitivity and selectivity of NOx sensors currently available is of particular concern given the low levels of NOx in the engine-out emissions from internal combustion engines. For example, it may be expected that a typical gasoline combustion engine would produce levels of NO in the region of up to 2000 ppm, and levels of NO₂ in the region of up to 200 ppm. However, to be effective within an advanced combustion system, which may generate NOx emissions of less than 10 ppm, such a sensor must be even more sensitive to NOx. Other drawbacks of NOx sensors currently available for use in motor vehicle exhaust systems, relate to: response time, in view of the frequency with which engine conditions and engine emissions may vary; reliability; and cost.

In use, if an NOx sensor detects too high a concentration level in the engine-out emissions: (i) a feedback signal may be used to control e.g. EGR, in an attempt to adjust the amount of NOx produced; and (ii) a feedforward signal may be used to optimise the catalytic conversion (reduction) of NOx and/or purging (regeneration) of any NOx traps that may be used.

Oxygen (O₂) Sensors

Oxygen sensors (O₂ sensors, lambda probes, or lambda sensors) can be fitted into the exhaust system of a motor vehicle to measure the concentration of oxygen in the engine-out emissions. It is then possible to calculate the AFR in the engine-out emissions. In this regard, an AFR meter and a wide band O₂ sensors are one in the same, since AFR is directly related to excess O₂ in the exhaust. The AFR can be used as an indication of whether the charge entering the combustion chamber is too rich or too lean for efficient combustion. Thus, once the AFR in the engine-out emissions is known, an engine control unit may be used to control the efficiency of the combustion process (i.e. to give the best fuel economy and lowest exhaust emissions). For example, by measuring the proportion of oxygen in the exhaust gas (and by knowing other relevant parameters, such as the volume and temperature of the air entering the combustion cylinders), an engine control unit may, for example, use look-up tables or a computer model to determine the optimal amount of fuel that should be injected into the combustion cylinder to completely combust with the available oxygen (i.e. the optimal stoichiometric ratio of air to fuel can be determined).

A typical oxygen sensor is made from a ceramic cylinder (such as zirconium dioxide), which is plated inside and out with porous platinum electrodes. This may then be encased in metal gauze. The sensor provides an electrical signal in proportion to the difference in O₂ concentration between the engine-out emissions (i.e. the environment to be measured) and the external air. Generally, the sensors are required to be at a temperature of approximately 300° C. in order to work effectively, and therefore, the sensor may include its own heating element.

The above type of oxygen sensor is known as a “narrow band” oxygen sensors, which means that the measurement of oxygen concentration is extremely crude. In fact, the measurement is essentially binary, allowing only a reading of “lean” (i.e. high AFR), or “rich” (i.e. low AFR); so that the engine control unit tends to adjust the combustion process cyclically between these two extremes. In addition, these sensors can be slow to respond, because they average the signals over a period of time. Another type of narrow band oxygen sensor is made of titanium oxide. These type of sensors are largely irrelevant to diesel engine control systems, because, by way of example, “rich” and “lean” are relative terms, and the term rich when used to describe diesel AFRs is likely to refer to an AFR that is still quite lean in relation to other engine types.

An improved type of oxygen sensor is known as a “wide band” oxygen sensor. It comprises a planar zirconium oxide element and an electrochemical gas pump, which (by way of a feedback loop) controls the gas pump current to keep the output of the electrochemical cell constant. In this way, the gas pump current directly indicates the oxygen content of the exhaust gas. Accordingly, the sensor can determine the oxygen content far more rapidly (without the need for averaging), and allows the engine control unit to more effective control the combustion process. These sensors are not yet in wide use in motor vehicles.

The optimal position for an oxygen sensor in a motor vehicle is between the combustion cylinder and the CC, when used. However, a sensor may also be used after the CC.

As previously noted, these sensors cannot be used to accurately determine or reduce engine-out emissions of NOx.

Hydrocarbon (HC) sensors

As already noted, an HC sensor can provide a means for determining the efficiency of the combustion process, since the higher the concentration of HCs in the engine-out emissions, the more unburned fuel is present, and therefore, the less efficient is the combustion process. Once the concentration of HCs in the engine-out emissions is known, an engine control unit can provide a feedback signal to alter one or more of fuel injection quantity, fuel injection timing and EGR, so as to improve the efficiency of combustion.

A conventional hydrocarbon sensor comprises: a thin, solid electrolyte layer having high proton conductivity; a pair of electrodes, which sandwich the proton conducting layer; and a catalytic surface on the anode, which liberates protons from HCs. Protons liberated from HCs travel through the electrolyte layer under the influence of a (constant) potential difference across the electrodes, and the current measured is proportional to the concentration of HC at the anodic surface. HC sensors are described, for example, in U.S. Pat. No. 6,103,080, U.S. Pt. No. 6,238,535 and U.S. Pat. No. 6,037,183.

Various chemical reactions can be used for the generation of hydrogen from HCs, including dehydrogenation, cracking, and steam reforming.

Commercially available compounds such as Fe₂O₃ or FeO(OH) may be used as dehydrogenation catalysts. Another suitable dehydrogenation catalyst is LaFeO₃, which has better stability than some other Fe-containing catalysts. Precious metals, such as Pt or Pd may also be used as dehydrogenation catalysts (when supported on porous ceramic materials, such as MgO, Al₂O₃, or a silica gel). However, these materials may be less selective that the Fe-containing catalysts. Cracking catalysts are compounds such as La_(1-x)Ce_(x)FeO₃ (e.g. La_(0.9)Ce_(0.1)FeO₃), while steam reforming catalysts includes materials such as NiO, nickel metal and precious metals.

The catalyst material in the form of a powder is typically mixed with an organic solvent to form a slurry and applied to the surface of the sensor electrode.

As electrodes in HC sensors, numerous materials may be used, including Pt, Pd, Ag, Au, or their alloys, and certain metal oxides.

The thin solid electrolyte layer can be a Ba-Ce-based oxide layer.

The selectivity (i.e. cross-reactivity) of HC sensors is of some concern. For instance, although a conventional HC sensor can be capable of linearly responding to HC concentration in the range of several percent down to a few ppm; at low concentrations of HCs (e.g. less than 10 ppm), the sensors may be significantly confused by oxygen in the atmosphere. This problem is of particular concern when monitoring HC concentration in the engine-out emissions of advanced combustion engines, which operate with lean fuel-air mixtures. In addition to oxygen effects, HC sensors tend to be sensitive to hydrogen. Therefore, especially at low concentrations of HC, an HC sensor may give quite inaccurate measurements of HC level.

Alternative types of HC sensors, such as fibre optic devices, are under development (e.g. US 2006/165344). These sensors should be less sensitive to non-HC contaminants.

A further limitation of HC sensors for use in controlling engine combustion is that, as already noted, there is not such a reliable relationship between HC and NOx concentration in engine-out emissions, as there is between NOx and CO, as demonstrated herein. Therefore, although an engine control unit may adjust the combustion cycle in response to engine-out HC levels, this system may not be capable of accurately adjusting NOx levels.

Ammonia (NH₃) sensors

Ammonia (NH₃) sensors may be used in a feedback system for controlling Selective Catalytic Reduction (SCR); in particular, such a sensor may be used downstream of a CC to monitor the amount of NH₃ in exhaust gas emissions, and report to an engine control unit in the event of “ammonia slip” (i.e. too much urea injected during SCR). There are a wide variety of NH₃ sensors, including: solid-state sensors, semi-conductor sensors, electrochemical sensors and optical (e.g. infra-red) sensors (some of which are discussed in US 2006/194330 and U.S. Pat. No. 6,069,013, for example).

Solid-state sensors provide high reliability, relatively low cost, and can measure NH₃ in the range from low ppm range to several percent. However, some can be quite sensitive to other gases (e.g. O₂) in engine-out emissions, and are, therefore, impractical for monitoring engine-out emissions in motor vehicles. Semi-conductor sensors are typically based on materials such as metal oxides or polymers, and measure the change in resistance or capacitance of the coating as a function of adsorbed molecules. They are capable of measuring NH₃ concentrations in the low ppm range. However, a problem with semi-conductor oxides is the issue of cross-contamination, since most gases adsorb onto high-surface area ceramic substrates to some extent. In engine-out or exhaust gas streams the main problem of cross-contamination is with CO and NOx. Meanwhile, a disadvantage of polymer-based sensors is that they can only be used at relatively low temperatures, at which they are chemically stable. A further problem with these sensors is that gas absorption is kinetically relatively slow and can be inhibited at high temperatures, which means that these sensors can have a slow response time and are not best suited for monitoring engine-out emissions.

Electrochemical sensors can provide a high degree of gas specificity. An example of an electrochemical sensor is a mixed-potential based ceramic sensors, which comprise an oxygen ion conducting membrane and metal, metal oxide or perovskite sensing electrodes. Their high temperature operating capabilities can make them suitable for use in exhaust/engine-out emission streams. Optical sensors (such as infra-red sensors) provide high gas specificity (i.e. little cross-sensitivity), stability and durability. However, disadvantages include a potentially long lag period for obtaining NH₃ measurements, expense and low operating temperature range.

A limitation of all NH₃ sensors is that levels of NH₃ cannot be directly linked to levels of NOx, and so NH₃ sensors have little use in feedback systems for controlling the combustion process.

Measuring CO in Engine-Out and Exhaust Gas Emissions

As described herein, it has now been found that the combustion process of an internal combustion engine, and particularly in an advanced combustion system, can be monitored, modelled and controlled by measuring the concentration of CO in engine-out emissions. Most advantageously, it has now been found that CO concentration in engine-out emissions from an advanced combustion engine can be used to: (i) monitor the combustion process and, therefore, by appropriate adjustments of engine parameters, to control the combustion process; and (ii) estimate or determine the concentration of NOx in engine-out emissions.

With particular regard to point (i) above, by using the measurement of CO in engine-out emissions to control an advanced combustion system (for example, using an engine control unit), it is also possible to: (a) control combustion efficiency; (b) optimise the combustion process for the minimisation of NOx production (i.e. the level of NOx in engine-out emissions directly resulting from the combustion process) using a feedback mechanism; and (c) optimise the removal of NOx from engine-out emissions, for example, using feedforward mechanisms to control exhaust after-treatment systems.

In further regard to point (ii) above, the use of CO concentration as an indication of NOx levels, and to adjust the combustion process to reduce NOx levels, offers significant advantages over prior art systems, because inter alia the measurement of CO in engine-out emissions can be more reliable, less expensive and more accurate than the available means for measuring the concentration of alternative gases in engine-out emissions.

Carbon Monoxide (CO) Sensors

Carbon monoxide (CO) sensors can be made from a range of materials, including base metals such as titanium, aluminium and copper. The configurations of CO sensors currently in industrial use are relatively simple and straightforward, and therefore, the manufacture of a CO sensor for use in a control system for an internal combustion engine can be considered to be technologically unchallenging. CO sensors in common usage are primarily used from monitoring atmospheric CO in the home or workplace, can be broadly classified into the following groups: (i) biomimetic sensors; (ii) semiconductor sensors; and (iii) electrolytic/electrochemical sensors; and (iv) spectroscopic sensors.

Biomimetic (or chemical) CO sensors (e.g. U.S. Pat. No. 5,063,164; U.S. Pat. No. 5,618,493) mimic the effect that CO has on haemoglobin in the human body. In gel cell biomimetic sensors, a gel-coated disc will change colour (e.g. darken) in the presence of CO. For example, these devices are usually based on palladium or iodine salts, which exhibit a colour change on exposure to CO. The presence of CO gas may then be determined by monitoring the optical properties of the sensor, for example, by visual inspection, or using a sensor that detects colour change. Typically, the detector system includes a housing containing a photon source, which emits photons in a region of the electromagnetic spectrum that the sensor absorbs in response to CO exposure, and a photodetector that measures light at the corresponding wavelength. These detectors can be inexpensive, particularly with respect to the lower technology devices, but tend to have slow response times. Although, at present, they may be more useful for monitoring CO emissions in the home, some systems may also be suitable or adaptable for measuring CO in engine-out emissions.

Semiconductor sensors use an electrically powered sensing element, which can be monitored by a computer. The sensing element typically comprises a thin layer of SnO₂ overlaid onto a ceramic base, which contains electrically conductive wires. The ceramic base does not conduct electricity, but electrons are able to travel over the surface of the SnO₂ layer. Absorption of CO onto the surface of the SnO₂ causes a flow of electrons between the electrical wires, which is proportional to the concentration of CO. Generally, this type of sensor operates in cycles in which CO is first detected and then the CO absorbed in that stage is burned off the sensor.

These solid-state semiconductor sensors can be highly accurate and reliable, making them suitable for monitoring CO concentration in motor vehicle engine-out emissions and/or exhaust gas emissions. Although these sensor typically require heating to operate, this is less of a disadvantage in the context of measuring CO levels in a hot engine-out emission stream.

Another type of semiconductor CO sensor contains both a p-type semiconductor (CuO) and an n-type semiconductor (ZnO) in physical contact with each other (Japanese Patent Laid-open No. Sho-62-90529). This type of sensor is reported to have improved selectivity for CO, but its mechanism of action can mean that its sensitivity is prone to variability.

It will be appreciated that the methods of the present invention benefit from the use of a CO sensor that is highly selective for CO over other gases in engine-out emissions. Sheikh et al., (supra) briefly discuss a CO sensor that may have practical application for monitoring CO levels in combustion emissions, which contain NOx, CH and O₂. This sensor is based on p-n hetero-junctions between anatase (n) and rutile (p) with a TiO₂ base material. In addition, the anatase portion was doped with CuO (to catalyse CO combustion) and La₂O₃ (to stabilise the anatase phase of TiO₂).

Electrolytic (or electrochemical) sensors usually contain three electrodes (typically made from platinum) that are placed into an electrolytic solution. CO is adsorbed into the solution and oxidised at an electrode, generating an electrical current that is proportion with the CO concentration. In some cases, the sensitivity of these devices can be affected by the adsorption of other gases, such as NO and O₂ into the electrolytic solution. However, generally, these sensors provide a high degree of accuracy and can be extremely specific for CO.

Spectroscopic sensors take advantage of the fact that CO absorbs infrared radiation. Thus, infrared CO sensors measure the differences between infrared radiation absorption in a test cell containing a gas to be tested (e.g. an engine-out emission gas) and a reference cell containing a Known composition of gas. A disadvantage of these sensors is that, at present, they tend to be expensive and bulky, requiring a long path length, heated IR sources, and expensive detectors. However, they are accurate and sensitive, so that they may offer potential for use in motor vehicles.

In summary, the electrolytic and spectroscopic detectors provide the advantages of rapid response times, high resolution and high accuracy. However, compared to biomimetic and semi-conductor sensors, they tend to be expensive and less suitable for domestic use, such as in a motor vehicle. In the methods and systems described herein, the CO sensor used for measuring CO concentration in engine-out emissions and/or exhaust gas emission streams may be any suitable CO sensor known to the person of skill in the art, and not limited to the devices described herein. Preferably, however, the CO sensor used is a semi-conductor sensor, because these sensors are well known in the art and, presently, offer the most optimal combination of features for domestic use (such as in a motor vehicle). As the technology of CO sensors develops, however, it is anticipated that other types of CO sensors may become more useful for measuring CO levels within the engine-out emissions and exhaust gas emissions from an internal combustion engine. This is particularly the case, because, until now, there has been no demand for a CO sensor for monitoring engine-out emissions within the exhaust system of a motor vehicle. However, it is anticipated that a suitable CO sensor could be readily designed and manufactured by a skilled person in the art. The methods and systems described herein encompass the use of any such CO sensing equipment that may become available.

Engine-Out CO Concentration in Control of Advanced Combustion

As would perhaps be expected in a finely tuned chemical-mechanical process, such as advanced combustion; small changes in any of the important variables could have profound effects on the result of the process. Advanced combustion is a finely tuned process, the primary aim of which is to minimise the production of NOx, thereby to reduce levels of atmospheric pollution from internal combustion engines.

Advanced Combustion Instability

As already discussed, present and future emissions regulations on exhaust gas emissions are extremely challenging to meet without the use of effective after-treatments, such as CCs. It is desirable to reduce or eliminate the reliance on these after-treatments and, therefore, engine manufacturers also seek to reduce those emissions at source, i.e. by optimising the combustion process itself. However, as demonstrated in FIG. 1, the pursuit of low engine-out NOx emissions can challenge the thresholds for stable combustion.

FIG. 1 demonstrates the effects on: (i) the coefficient of variance (COV) of “mean effective pressure” (MEP) within a combustion cylinder, and (ii) engine-out HC emissions, which result from pushing the combustion process in the direction of minimal NOx emissions. In the example shown, minimal NOx emissions for a particular engine load (in this case: steady state operating condition of 1200 rpm, 59 Nm on a North American V8 Diesel engine capable of Tier II Bin 8 emissions levels for a 6000 lb vehicle), may be achieved by, inter alia, a combination of EGR rate, injection timing, and boost pressure.

By way of explanation, the MEP within a combustion cylinder provides a measure of the percentage load at which the engine is operating (i.e. it is an indication of the torque that is being delivered by the engine). Thus, a variation in MEP from one event to the next is an indication that the engine is misfiring and that the combustion process is not operating efficiently. Accordingly, a consistent/reproducible combustion process should lead to a predictable MEP for each combustion cycle and a low COV. In the example depicted it can be seen that at low NOx levels the COV of MEP increases dramatically, which indicates that the combustion cycle is lacking control and reproducibility under the conditions of EGR etc. that are necessary to reduce NOx. Similarly, it is apparent that engine-out HC emissions also increase rapidly under these conditions of low NOx emissions, which demonstrates that a highly inefficient combustion process is taking place.

From this data it appears that even a slight error in EGR rate, injection timing, boost pressure, or other combustion-controlling processes could lead to highly inefficient combustion, which may reduce NOx emissions, but with unacceptable losses in terms of fuel economy and other relevant factors.

Relationship Between NOx and EGR

A key challenge with advanced combustion is consistency of combustion from one combustion cycle to the next. Without this capability, the engine-out emissions of NOx, and other undesirable gases, could fluctuate dramatically and severely hamper attempts to provide consistently low NOx emissions. For example, it is known that even small errors in air charge (i.e. the mass of air in a cylinder immediately preceding combustion), or combustion chamber temperature can lead to drastic differences in engine emissions, combustion stability, and combustion noise. As described above, changes in air charge and combustion chamber/fuel temperature can be achieved by changes in EGR; i.e. the amount of exhaust gas that is retained in or recirculated to the combustion chamber from one combustion cycle to the next.

FIG. 2 graphically demonstrates the relationship between NOx emissions from an advanced combustion diesel engine as a function of the amount of EGR for two different engine loads (data from a North American V8 diesel engine capable of Tier II Bin 8 emissions in a 6000 lb vehicle). From the graph it can be seen that relatively large changes in NOx emissions are caused by relatively small deviations in EGR. For example, an approximate 5% change in EGR may double, or even triple, the engine-out NOx emissions. This effect is especially pronounced at high engine loads.

As discussed above, there is no precise method for controlling the amount of EGR during advanced combustion, although improved systems of controlling EGR are in development. Hence, it can be difficult to control EGR to within 5%. (Even in the case that EGR is accurately controlled, NOx emissions can still be variable due to AFR, temperatures, pressures and general engine-to-engine discrepancies, if not properly controlled). Therefore, it can be quite challenging to control and maintain minimal levels of NOx by adjusting EGR exclusively. It is particularly difficult to achieve consistent combustion parameters by directly measuring NOx and adjusting EGR in response (so as to control NOx), because of the highly sensitive relationship between NOx and EGR, and the relatively low levels of NOx that are produced. In this regard, at levels of NOx of e.g. 5 ppm, an NOx sensor may be near to the limit of its resolution and, therefore, prone to error. However, if a consequential small change in another combustion parameter, such as EGR, is made in response to an apparently small error in an NOx measurement, this could cause an overly large compensation in actual NOx levels, because as indicated in FIG. 4, a small change in EGR results in a much larger proportional change in NOx.

It would be advantageous, therefore, to have a system for controlling a combustion process in an internal combustion engine that was not reliant on such highly sensitive and variable parameters such as measuring NOx. Measurement of CO levels in engine-out emissions and its use for controlling combustion provides such a system as described below with reference to FIGS. 5 and 7.

Cetane Variance in Advanced Combustion

As has been discussed briefly above, it is known that the type of fuel used in an internal combustion engine can have a significant impact on the results of the combustion process itself. Thus, the control of advanced combustion is not merely a matter of adjusting air charge and other engine controls, but is also dependent on fuel properties. The most notable of these fuel properties is cetane number, which represents the fuels capacity to ignite (i.e. combustibility).

FIG. 3 demonstrates the variation in combustion—indicated as a function of NOx emissions—in relation to fuel injection timing, for three fuels with different cetane numbers (CN). Results are shown for fuels having CNs of 25 (open circles), 40 (open triangles), and 53 (filled circles), and the injection timing is indicated as a function of crankshaft angle in advance of top-dead-centre (ATDC).

First, the data depicted clearly demonstrate that changes in CN can cause significant variations in engine-out NOx emissions; i.e. at higher CNs the fuel produces higher levels of NOx emissions. In addition, it was shown from measurements of in-cylinder pressure and heat release rate, that the more ignitable fuels (i.e. those having higher CNs), combusted earlier in the combustion cycle, for an identically timed fuel injection event, than the less combustible fuels (data not shown). Thus, it is apparent that fuel CN affects both combustion and engine-out emissions of NOx.

Further in this regard, it is notable that, generally, the CN of commercially available diesel fuel in North America can vary from 40 to 50, depending on the region and the season. Thus, the effects observed for changes in CN are relevant to motor vehicles in common usage. Moreover, where consumers are unable to control the specific parameters of the fuel type (e.g. CN) that is used, it is a separate and significant challenge for an engine to detect the type of fuel present, and to adjust its combustion process accordingly.

It would be advantageous, therefore, to have a method and system for controlling combustion in an internal combustion engine (and especially in an advanced combustion engine), which is not sensitive to, or affected by, variations in fuel CN. The methods described herein provide such as system, as will be discussed.

Relationship Between CO and NOx

The process of advanced combustion is designed primarily to improve fuel efficiency and to reduce engine-out NOx levels, because emissions regulations related to NOx are extremely strict (limiting allowable NOx levels to virtually zero). The emissions levels of other undesirable pollutants, such as CH and CO tend, therefore, to be a secondary issue at the combustion stage, such that any reduction in CO that is required may be carried out mainly at the stage of after-treatment. In fact, it seems that the advanced combustion conditions that favour a reduction in NOx, tend to favour the production of CO. In this study, the relationship between engine-out NOx and engine-out CO levels was investigated in an advanced combustion engine.

As shown in FIG. 4, it has now been found that there is a clear relationship between engine-out NOx concentration and engine-out CO concentration in an advanced combustion engine. In more detail, at NOx concentrations of between approximately 40 ppm and 20 ppm, a decrease in NOx concentration is reflected by a gradual increase in CO concentration. At lower levels of NOx emissions, for example, between 15 ppm and 5 ppm, a reduction in NOx concentration is reflected by a more rapid increase in CO emission levels. The absolute concentration of CO in engine-out emissions is also dependent on engine load, as indicated by the data for engine loads of 1200 rpm, 59 Nm (solid line); 1600 rpm, 50 Nm (dashed line); and 1600 rpm, 130 Nm (dotted line). Thus, these data demonstrate that for a given engine load (set of engine operating conditions), the concentration of engine-out NOx can be determined from the concentration of engine-out CO.

Furthermore, it is notable that whilst the concentration of NOx varies between approximately 5 ppm and 50 ppm (0.0005% to 0.005%), which range can be extremely difficult to measure accurately; the concentration of CO tends to vary between 0.1% and 1% (1000 ppm to 10000 ppm). Hence, concentrations of CO in engine-out emissions may be in the order of 200-fold higher than concentrations of NOx under the same conditions. Moreover, from the graph of FIG. 4, it is apparent that a change in NOx concentration of just 1 ppm (i.e. 0.0001%) can lead to a change in CO concentration of 0.1% (i.e. 1000-fold greater). Therefore, it appears that measurement of CO concentration can offer much improved resolution and accuracy over NOx measurement.

In summary, using conventional gas sensing equipment, it may be less technically challenging and less expensive to measure accurately the concentration of CO in engine-out emissions, rather than the concentration of NOx. Moreover, as demonstrated, the measured concentration of CO can provide a direct link to NOx concentration, so that the concentration of NOx can be accurately assessed.

Accordingly, the use of CO sensors to monitor the engine-out (and exhaust gas) emissions of an internal combustion engine, and particularly a diesel advanced combustion engine, may provide the means for a reliable system for optimising advanced combustion to reduce engine-out NOx levels. In short, CO sensors may be used to: (i) rapidly monitor the combustion process in real time; (ii) accurately assess the combustion process in regard to the production of NOx; and (iii) sensitively adjust the combustion process to reduce NOx levels.

Beneficially, in the methods and systems of the invention, a CO sensor will be arranged so as to measure the concentration of CO in engine-out emissions from an internal combustion engine. Thus, a CO sensor will preferably be located in the exhaust system downstream of one or more combustion cylinders and upstream of any after-treatments, where used. Preferably, the methods provided herein obviate or reduce the need for after-treatments to reduced engine-out levels of NOx. Where one or more after-treatments are employed, a CO sensor may optionally be located in the exhaust system downstream of said after-treatments, so as to monitor the exhaust gas emissions released into the atmosphere.

Similarly, in the methods and systems described herein, other means of monitoring exhaust gas streams may be used. For example, one or more sensors for measuring concentrations of CH, O₂ and/or NOx may be arranged to measure the concentration of said gas in the engine-out or exhaust gas emissions. Preferably, the methods and systems of the invention obviate the need for such sensors to monitor engine-out emissions. However, it may be advantageous to measure the concentrations of certain exhaust gases, such as NOx, in the exhaust gas emissions, e.g. downstream of any after-treatments, so as to assess the functioning of said after-treatments, when used. When after-treatments are not used, or are used sporadically, it may then be advantageous to include one or more auxiliary gas sensor, such as an NOx sensor, in the exhaust system to measure engine-out emission levels of the gas of interest, so as to validate the correct functioning of the combustion process. It will be appreciated that the concentration of CO measured downstream of one or more after-treatments may have no correlation with, or an entirely different correlation with the concentration of NOx either upstream or downstream of the after-treatment.

Preferably, the concentration of CO in the engine-out emissions is measured continuously, or semi-continuously over the period during which the engine is running. By semi-continuously, it is meant that the CO sensor monitors CO concentration as frequently as its mode or operation and technology allows. For example, separate measurements of CO concentration may be taken every minute or more frequently, such as 2, 3, 4, 5, 6, 10, 20, 30, 60 or more times per minute.

The concentration of CO that is measured by the one or more CO sensors is preferably recorded/monitored and processed by a computer, such as an engine control unit. An engine control unit (or ECU) preferably obtains a set of data for each CO measurement recorded and, by way of a feedback system, adjusts one or more of the engine parameters associated with the control of combustion, accordingly.

For example, the data may provide a set of combustion parameters for optimising the combustion cycle in view of the measured CO concentration. Preferably, the engine load is also taken into account for each CO concentration measured. In particular, the measured concentration of CO is preferably used by an engine control unit to determine: the optimal EGR, and/or the optimal injection timing, and/or the optimal fuel quantity to be injected in each combustion cycle, so as to minimise the production of engine-out NOx. In addition (or alternatively), the CO concentration measurements can be used to predict or monitor the NOx levels in the engine-out emissions. Such NOx measurements can, for example, be used in a feedforward system to control/adjust any after-treatments used to reduce optimally the NOx gases in the engine-out emissions. Most preferably, the engine control unit also takes account of the relevant engine load for each CO measurement.

Any suitable means of correlating the measured concentration of CO with one or more other engine and/or combustion parameters may be used. Thus, data in the form of a graph, such as that depicted in FIG. 4, may provide a “calibration curve”, from which a concentration of NOx can be determined for any particular value of CO concentration. However, in all cases where one form of data is used to calculate, predict or select other parameters, it is not intended that the means of data comparison should be restricted to a graph or other means of pictorial display. Instead, it is intended that any means for data correlation; including graphs, tables and models, and any suitable means by which such data may be digitally stored and processed (e.g. by way of a mathematical equation), which provides a means of converting a measurement of CO concentration into a corresponding NOx concentration or one or more optimal combustion parameters, is encompassed. Preferably, the data correlating means has been previously generated, for example, by carrying out a prior series of tests or models to obtain sample measurements, to provide an existing set of reference data.

Preferably, the means of data correlation is electronically stored by way of a “look-up table”, mathematical equation or model that provides, for example, a preferred fuel injection timing and/or level of EGR for an inputted CO concentration measurement. Advantageously, more than one means of data correlation is available and has been previously generated, for each of a plurality of different engine loads. For instance, it is beneficial to produce a series of (standard) data sets of engine-out CO concentration against NOx concentration and/or preferred (optimal) EGR, and/or optimal fuel injection timing, and/or optimal fuel injection quantity for a series of different engine loads.

More preferably, the means of data correlation is one or more look-up tables (preferably in electronic form) that correlate CO concentration to: (i) levels of engine-out NOx at particular engine loads; and (ii) optimal engine parameters for controlling engine combustion so as to reduce engine-out NOx levels, where possible.

Feedback means for adjusting one or more engine parameters is preferably provided, and/or feedforward means for adjusting one or more after-treatments is also provided.

By way of non-limiting example, in one aspect of the invention, a series of combustion experiments is performed on a particular engine type under known engine and combustion conditions/parameters. The amount of EGR is varied in 1 or 2% intervals from a minimum level of 0% EGR (or approaching 0%) to a maximum level of e.g. 75%. It will be appreciated, however, that a maximum level of EGR of e.g. 60%, 80% or 90% may also be suitable. For each EGR level the concentration of CO and the concentration of NOx in the engine-out emissions is measured. These data points can be used to produce a calibration curve, look-up table, or model to correlate/link a particular CO emission level to a particular NOx emission level. Thereafter, by measuring CO concentration under the test set (or a closely related set) of conditions, the concentration of NOx in the emissions can be calculated (or estimated). The experiment can be repeated under different sets of conditions to obtain further data sets (look-up tables, calibration curves or models etc.), which can be used to calculate engine-out NOx concentration from a measured engine-out CO concentration.

Conditions that may be varied in order to build up a substantial set of data, include: oil temperature, in-cylinder pressure, EGR, AFR, injection timing, fuel injection quantity, engine type, fuel type (e.g. petrol/diesel), engine speed, engine load and other parameters known to the person of skill in the art.

These experiments have the further advantage of providing information of the engine and/or combustion parameters that are required in order to produce specific concentrations (or concentration ranges) of NOx in the engine-out emissions.

Thereafter, when an engine is in use, by measuring the concentration of CO in the engine-out emissions, not only can the level of NOx in the engine-out emissions be calculated, but also, a set of engine and/or combustion conditions may be calculated (or determined) to adjust the engine-out concentration of NOx to a particular (predetermined) concentration or concentration range. An engine control unit may then use this information to feedback instructions to control or adjust the engine and/or combustion parameters, for example, to reduce the concentration of NOx produced in the combustion process.

Furthermore, an engine control unit may use the calculated concentration of NOx to feedforward instructions to adjust or control the activity of one or more exhaust after-treatments (when present), to reduce the level of NOx in the engine-out emission, before the exhaust gases are released into the atmosphere. This feedforward system is particularly preferred in cases where the engine control unit is not able to reduce the engine-out emissions of NOx to below an allowable emissions level, without the use of one or more after-treatment.

In addition, it is an option to disable the correlation means between engine-out CO and engine-out NOx, when the engine is operating under conditions in which the correlation between CO and NOx concentration breaks down. For example, this may be appropriate when the engine is operating outside of the conditions for advanced combustion. In such circumstances, it is preferred that the engine control unit of a motor vehicle adjusts the activity of after-treatments, where used, to ensure that exhaust gas emissions of NOx do not exceed allowable emissions levels.

Relationship Between CO and Combustion Stability

As advanced combustion is pushed in the direction of minimal NOx production, combustion stability can become a significant issue (FIG. 1). Combustion instability can lead to poor combustion and therefore, a significant amount of unburned fuel at the end of a combustion cycle. This unburned fuel is manifested by the presence of HCs in engine-out emissions. Thus, as combustion becomes less stable, higher levels of HCs can be measured in engine-out emissions. The disadvantages of unstable combustion may include a reduction in fuel economy, a loss in engine power and increased engine noise.

FIG. 5 demonstrates that during advanced combustion there is a correlation between engine-out CO emissions and engine-out HC emissions. Thus, the level of CO emissions also provides a link to combustion stability. Moreover, it can be seen from FIG. 5 that measuring CO emissions provides excellent resolution with respect to engine-out HC levels. In this regard, a change in HC emission of 100 ppm (0.01%) can result in a change in CO concentration of approximately 0.1% CO. In other words, once the relationship of CO emissions to HC emissions is known, measurement of CO concentration in engine-out emissions provides a highly sensitive measure of engine-out HC concentration and hence, provides a sensitive measure of combustion stability and fuel economy.

Thus, in one aspect of the invention, the measurement of CO concentration in an engine-out emission is regarded as a indication of engine combustion efficiency, and this measurement is used in a feedback system to adjust one or more engine parameters in order to control the combustion process. In this way, measurement of CO concentration provides a means for ensuring that combustion parameters are not selected that would result in unstable combustion, and moreover, the control of the combustion process does not rely on a knowledge or even a prediction of the actual concentration of NOx in the engine-out emissions. A further advantage of this aspect of the invention is that optimising the combustion process on the basis of engine-out CO levels has the benefit that engine-out NOx levels are inherently reduced (see FIG. 4).

In more detail, the relationship between the amount of EGR and the level of NOx emissions demonstrated in FIG. 2 provides the possibility of minimising engine-out NOx levels irrespective of whether the absolute concentration of NOx in the engine-out emission is known, by achieving an optimal EGR level. Furthermore, at a particular engine speed and load there is a maximum EGR that can be used before combustion stability breaks down, as indicated in FIG. 1 by the rapidly increasing COV of IMEP with decreasing NOx concentration (resulting from increasing EGR). However, to effectively control the combustion process by relying on these relationships alone, it is necessary to make direct measurements of NOx and/or EGR and to implement sensitive adjustments to each parameter.

In the advantageous method of the invention, the relationship demonstrated in FIG. 5 and the measurement of CO concentration in engine-out emissions is used to assess when combustion stability is at the verge of degrading to the point at which fuel economy and HC emissions have been unacceptably compromised, and control the combustion process accordingly.

As a first step, it is advantageous to provide one or more calibration curves, look-up tables, or models to correlate/link a particular CO emission level to a corresponding HC emission level. Since CO levels in engine-out emissions can be dependent on engine speed and load, it is advantageous to determine a desirable “target” CO concentration level (or a corresponding maximum desirable HC emission level, or COV of MEP) for one or more engine speed and load conditions. The target CO concentration is the level of CO which provides the minimum possible NOx concentration while combustion stability and fuel economy are maintained within acceptable parameters. These determinations, for example, the minimum desirable fuel consumption/economy at a particular engine speed and load, and a particular acceptable level of combustion stability can be carried out during engine development and testing (or at any point thereafter). The appropriate parameters may then be recorded in an engine control/management unit for implementation when the engine is in use.

Expressed in another way, it may have been predetermined that, for a particular engine speed and load, the maximum level of HC in the engine-out emission before combustion stability and fuel economy are unsatisfactorily compromised is 400 ppm. By cross-reference to an appropriate means of data comparison, for example, a look-up table, calibration curve or equation (such as a graph similar to that shown in FIG. 5), the concentration of CO in the engine-out emission that will achieve the predetermined level of HC can be readily obtained. The corresponding CO concentration can be considered to be a “target” CO concentration for the same (and similar) engine conditions.

Where engine testing to obtain a suitable means of data comparison (calibration curves, look-up tables and so on) have not been carried out under the specific engine conditions in use, then an engine management unit may operate in one (or a combination) of two ways. For example, the data obtained under the most similar engine operating conditions (e.g. speed and load) to those in use may be taken as an approximation of the combustion conditions in use. In another method, a regression analysis or algorithm may be used to adjust the data obtained under the closest engine operating conditions in order to obtain a modified set of data that is an estimate of the combustion parameters in use. In this way, an engine management unit and the methods of the invention can be used under all operating conditions of engine speed and load, despite that a means of data comparison, obtained for example, during engine development and testing, is not available for every possible set of engine operating conditions. These methods of approximating and/or estimating the engine combustion parameters in use apply similarly to all of the methods and systems of the invention, which involve the use of a means of data comparison.

Thus, in accordance with the invention, a CO sensor is used to measure the engine-out CO concentration. The measured concentration of CO is then compared to the target CO concentration. If the measured CO concentration is lower than the target CO concentration, EGR is increased (to increase HC concentration and inherently reduce NOx concentration); whereas if the measured CO concentration is higher than the target CO concentration, combustion is unstable (or inefficient) and EGR is reduced. By performing these steps advanced combustion can be controlled by measuring engine-out CO concentration to minimise NOx concentration within acceptable levels of, for example, combustion stability, engine-out HC concentration and fuel economy.

This aspect of the invention is further illustrated by reference to FIG. 7, described below.

In summary, CO is used to maintain combustion efficiency, allowing an engine control unit (ECU) or engine management unit to add maximum EGR without causing misfire, and consequently minimal NOx emissions are produced for the engine operating conditions in use. In fact, using this system it may not even be necessary to estimate (or measure) NOx concentration in engine-out emissions (in order to know that NOx levels are optimally reduced), but just to control EGR based on CO. Of course, estimating NOx based on CO levels may still be useful, as discussed below and elsewhere herein.

In an alternative method, the correlation between engine-out CO concentration and engine-out NOx concentration (FIG. 4) may be used to achieve a particular desirable NOx concentration by adjusting EGR (further described with reference to FIG. 9).

In this aspect, a series of combustion experiments is performed on a particular engine type under known engine and combustion conditions/parameters. The amount of EGR is varied in 1 or 2% intervals from a minimum level of 0% EGR (or approaching 0%) to a maximum level of e.g. 75%. It will be appreciated, however, that a maximum level of EGR of e.g. 60%, 80% or 90% may also be suitable. For each EGR level the concentration of CO and the concentration of NOx in the engine-out emissions is measured. These data points can be used to produce a calibration curve, look-up table, or model to correlate/link a particular CO emission level to a particular NOx emission level. Thereafter, by measuring CO concentration under the test set (or a closely related set) of conditions, the concentration of NOx in the emissions can be calculated (or estimated). The experiment can be repeated under different sets of conditions to obtain further data sets (look-up tables, calibration curves or models etc.), which can be used to calculate engine-out NOx concentration from a measured engine-out CO concentration.

Conditions that may be varied in order to build up a substantial set of data, include: oil temperature, in-cylinder pressure, EGR, AFR, injection timing, fuel injection quantity, engine type, fuel type (e.g. petrol/diesel), engine speed, engine load and other parameters known to the person of skill in the art.

These experiments have the further advantage of providing information of the engine and/or combustion parameters that are required in order to produce specific concentrations (or concentration ranges) of NOx in the engine-out emissions.

Thereafter, when an engine is in use, by measuring the concentration of CO in the engine-out emissions, not only can the level of NOx in the engine-out emissions be calculated, but also, a set of engine and/or combustion conditions may be calculated (or determined) to adjust the engine-out concentration of NOx to a particular (predetermined) concentration or concentration range. An engine control unit may then use this information to feedback instructions to control or adjust the engine and/or combustion parameters to correspondingly adjust the concentration of NOx produced in the combustion process. For example, on the basis of the measured CO concentration the amount of EGR may be increased to achieve a particular (lower) level of NOx in the engine-out emission.

Effect of Cetane Number (CN) on the Relationship Between CO and NOx

In FIG. 3 it is demonstrated that fuel CN can have a significant effect on the advanced combustion process. In particular, it has been demonstrated that, the higher the CN, the earlier the fuel—air mixture combusts and the higher the concentration of NOx in the engine-out emission. For this reason, and the fact that commercially available fuel can vary considerably in CN, it is widely believed that a method for CN detection/fuel analysis is necessary to adjust combustion parameters (e.g. fuel injection timing and fuel quantity), in order to maintain acceptable engine emissions and performance.

However, data from a recent study by the Oak Ridge National Laboratory has indicated that engine-out NOx emissions share a common relationship to engine-out CO emissions regardless of CN (Table 1). In brief, these data appear to demonstrate that for a constant engine-out CO emission level of approximately 2500 ppm, the engine-out NOx emissions will range only from 0.5 ppm to 1.7 ppm, for a range of different fuels having CNs of 34 to 76.

TABLE 1 Correlation between CN, engine-out CO emissions and engine-out NOx emissions in advanced combustion (Szybist & Bunting, “Cetane Number and Engine Speed Effects on Diesel HCCI Performance and Emissions,” SAE Technical Paper Series, 2005-01-3723). Cetane Number CO NOx CA50 Fuel (CN) (ppm) (ppm) (CA deg.) 1 76 2500 0.5 354 2 62 2500 1 358 3 48 2500 1 361 4 34 2500 1.7 372 5 19 2500 — —

These data suggest that if engine-out CO emissions are regulated, for example, using a feedback system controlled by an engine control unit, it will be possible to control engine-out NOx emission levels irrespective of the CN of the fuel. Significantly, such a system will obviate the need to know or estimate the CN of the fuel used, and thus, provides a further advantage of the methods and systems described herein.

Relationship Between CO and AFR

Advanced combustion is typically characterised by the use of high levels of EGR in comparison to other forms of internal combustion; although some work has been carried out on HCCI without EGR. EGR also effects the AFR in the combustion chamber/cylinder.

In the present investigations it has also now been found that the level of CO in engine-out emissions correlates to AFR, particularly as engine load increases. This correlation is demonstrated in FIG. 6, which provides values of AFR against CO emissions for a North American V8 Diesel engine capable of Tier II Bin 8 emissions (in a 6000 lb vehicle), under three different engine loads. The engine was rated at 720 Nm. In FIG. 6, the AFR is reported in units of AFRC, wherein the “C” stands for “carbon balance”. Several methods for measuring AFR are known to the skilled person in the art, and the method employed in this example used several emission analysers in a laboratory to count molecules of carbon (“C”, for example, CO, CH, CO₂, etc.). An alternative method is AFRO, which measures molecules of oxygen (wherein “O” stands for “oxygen balance”).

In this regard, at low loads of 1200 rpm, 59 Nm (filled squares, left-hand vertical axis) and 1600 rpm, 173 Nm (filled diamonds, left-hand vertical axis), it was found that the engine-out CO emissions were not only a function of AFR, but also of injection timing, injection pressure and EGR. Thus, the plots of AFR against CO emissions appear relatively more scattered, depending on these other variables. However, at 1600 rpm, 297 Nm (filled triangles, right-hand vertical axis), which is still less than 50% load, a strong correlation between engine-out CO emissions and AFR emerged, regardless of the other combustion variables.

Accordingly, the methods and systems described herein provide a means for determining the AFR in a combustion cylinder, and particularly during an advanced combustion process, on the basis of the measured concentration of CO in the corresponding engine-out emissions. The measured concentration of CO in the engine-out emissions may be correlated to the corresponding AFR using any appropriate means, as described above. Briefly, a calibration curve, look-up table, or mathematical algorithm/equation may be used to convert a CO concentration into an AFR value. Preferably, a plurality of “standard” sets of data is provided for correlating CO concentration to AFR for any of a number of different engine and/or combustion parameters, such as engine speed and load. The means of data correlation is preferably stored electronically, for example, so that an engine control unit can process the CO data measured, in real time, to determine AFR for the measured CO concentration.

Still more preferably, the methods and systems include a feedback means for adjusting combustion parameters, in view of the measured CO concentration and the corresponding AFR, to optimise the combustion process for: reduction of NOx emissions; reduction in particulate matter; injector corrections; and improvement of combustion efficiency.

Feedback and Feedforward Control of Advanced Combustion

As has been discussed above in detail, the methods and systems described herein provide important information on the combustion process in an internal combustion engine. This information, which is derived from the concentration of CO in engine-out emissions, can be used advantageously to: improve combustion efficiency (by feedback control); reduce the production of undesirable gases, such as NOx (by feedback control); and effectively remove undesirable gases, such as NOx, using after-treatments (by feedforward control).

FIGS. 7 to 10 exemplify means by which CO concentration may be used by an engine control unit to adjust: EGR; fuel injection timing; engine-out NOx emissions; and AFR, respectively, in order to optimise advanced combustion.

Turning to FIG. 7, for a given engine speed and load, a “target” engine-out NOx emission also equates to a “target” engine-out CO emission (as discussed hereinbefore, see e.g. FIG. 4). Using the feedback from an appropriately arranged CO emissions sensor, the error/difference between the target CO concentration and the actual (measured) CO concentration can feed a Proportional, Integral, Derivative [PID: a sum of 3 different corrections depending on error (Proportional), cumulative error (Integral), and change in error (Derivative)] for an appropriate modification to EGR. For example, where the measured CO concentration is lower than the target CO, the EGR would be increased to raise CO production. As has already been shown with reference to FIG. 4, the production of NOx in advanced combustion is reduced as the level of CO is raised. Therefore, it is not necessary to know or even estimate the concentration of NOx in the engine-out emissions at a specific point in time; it is sufficient to control EGR so as to achieve a predetermined CO engine-out emission level, knowing that the predetermined CO level will correspond to a desirable (optimal or target) NOx concentration.

It should be noted that EGR and advanced combustion may only apply to a portion of a given engine speed and load range; i.e. at very high engine loads it is known that present advanced combustion processes break down. Therefore, the P, I and D terms should preferably also depend on engine speed and load.

Likewise, advanced combustion may only be achieved effectively for a specific range of coolant and air temperatures. Therefore, the control system described may beneficially be disabled when the engine parameters are outside of the appropriate range.

The control loop does not have to be a PID. For instance, there could be a number of ways to modify EGR based on CO emissions. Furthermore, additional parameters may also be included for enabling/disabling the control, such as engine run time, oil temperature, or atmospheric pressure. Suitable additional parameters will be known to the person of skill in the art.

In addition, or in the alternative to the feedback system described above, the engine control unit may process CO concentration data for feedforward EGR demand. Thus, when the feedback system is disabled, e.g. due to parameters that fall outside of those required for advanced combustion, the algorithm of the engine control unit may preferably resort to a feedforward EGR demand, which may be based, for example, on engine speed, engine load, and various temperatures, pressures and other parameters, such as those mentioned above.

FIG. 8 is a schematic representation of an algorithm for adjusting fuel injection timing on the basis of a measured engine-out CO concentration.

If the CO emissions are high, there is an increased likelihood of poor combustion efficiency or misfire. One way of avoiding misfire is to advance fuel injection timing. Reducing EGR can also help avoid misfire, however, control of EGR has been discussed with respect to FIG. 7. Thus, to avoid misfire, a maximum engine-out CO emission level is identified (based, for example, on control measurements or modelling), preferably, for a range of engine speeds and loads. The maximum allowable CO level can be subtracted from the base target CO level, to obtain the maximum allowable “error” (or difference). Depending on the measured CO level, the exemplified algorithm is designed to advance injection timing in the event that the maximum allowed error is breached.

In a similar manner to the EGR control algorithm (FIG. 7), the engine-out CO emission measured by a CO sensor is fed into a PID or other control system. As before, the control terms are preferably a function of engine speed and load. Also, the control system may preferably be enabled or disabled according to engine speed and load, and various other combustion parameters, such as temperature and pressure (as discussed above).

Turning to FIG. 9, a schematic representation of an algorithm for estimating engine-out NOx levels is depicted.

It is important to know the engine-out NOx emissions from an internal combustion engine. Using the methods and systems described herein, the NOx emission levels can be calculated on the basis of the engine-out CO emissions. Various routes by which this calculation can be achieved may be apparent to the person of skill in the art. One such system is described herein. For example, first a “base” (or target) NOx emission level may be identified; preferably, for each engine speed and load. This base NOx emission level can then be correlated to a target CO emission level; and the “error” in the NOx emission level can be determined from the corresponding error between the target CO emission level and the measured (or actual) CO emission level. It may also be possible to represent NOx emissions entirely from correlated CO emissions.

As before, the algorithm may be disabled under certain conditions, for example, in cold conditions, or high altitude, and for specific engine speeds and loads where advanced combustion is not possible. Under these conditions where the algorithm cannot be operated, the engine-out NOx emissions may instead be estimated using open loop tables.

For after-treatment models it is also important to know the engine-out NOx emissions. Therefore, a similar algorithm to that in FIG. 9 may be used to provide a feedforward signal to control after-treatments. Typically, however, there would be one such algorithm, such as that exemplified in FIG. 9 which estimates engine out NOx, and those results would be used globally, for example: to adjust EGR in a feedback mechanism; and also in a feedforward after-treatment algorithm to control one or more exhaust gas after treatments. Such a feedforward system could, for example, up- or down-regulate an NOx-removing after-treatment on the basis of the calculated level of NOx in the engine-out emissions. In this way, the after-treatment could be operated optimally accordingly to the amount of NOx present, so that fuel efficiency is optimised and waste products/unnecessary exhaust emissions are minimised.

FIG. 10 depicts an algorithm for calculating AFR on the basis of engine-out CO concentration.

As discussed in regard to FIG. 6, AFR can be calculated directly from engine-out CO emissions. Preferably, the engine speed and load is taken into consideration when determining the AFR based on CO concentration. Once the AFR has been calculated, feedback systems may be employed to adjust the AFR, according to preferred parameters.

It will be appreciated that in any of the feedback systems described herein, the feedback signal from an engine control unit may adjust the engine and/or combustion parameters associated with one or more engine cylinder. Preferably, the feedback signal adjusts the engine and/or combustion parameters associated with each of the engine cylinders.

As before, there may be engine speeds and loads where the relationship between CO emission levels and AFR breaks down, for example, outside of the advanced combustion zone. Similarly, there may be low temperatures and atmospheric pressures under which the correlation between CO concentration and AFR is not appropriate. Under these conditions, preferably the algorithm is disabled, and in those circumstances, the levels of AFR are advantageously based solely on mass air flow (MAF-measured with an MAR sensor) and commanded fuel levels. 

1. A method for controlling an internal combustion engine to maintain a predetermined combustion efficiency, the method comprising: measuring the concentration of CO in an engine-out emission; determining whether the measured concentration of CO is above or below a target CO concentration for the engine under the operating conditions in use; and if the concentration of CO is above the target CO concentration, adjusting one or more engine and/or combustion parameters so as to reduce the concentration of CO in the engine-out emission, and if the concentration of CO is below the target CO concentration, adjusting one or more engine and/or combustion parameters so as to increase the concentration of CO in the engine-out emission; wherein the target CO concentration is determined on the basis of a correlation with indicators of combustion efficiency under known engine operating conditions.
 2. The method of claim 1, wherein said one or more engine and/or combustion parameters is EGR.
 3. The method of claim 1, wherein the known engine operating conditions include engine speed and load.
 4. The method of claim 1, wherein the predetermined combustion efficiency is selected during engine development and/or testing.
 5. The method of claim 1, wherein the target CO concentration is determined on the basis of a correlation with one or more engine and/or combustion parameters which are indicators of combustion efficiency; and wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and the one or more engine and/or combustion parameters.
 6. The method of claim 1, wherein the target CO concentration is predetermined on the basis of a correlation with one or more engine and/or combustion parameters; wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and the one or more engine and/or combustion parameters; and wherein the means of data comparison is a look-up table, calibration curve or equation obtained for the engine under known engine operating conditions.
 7. The method of claim 1, wherein the correlation with indicators of combustion efficiency under known engine operating conditions is an approximation of the correlation existing under the operating conditions in use.
 8. The method of claim 1, wherein the correlation with indicators of combustion efficiency under known engine operating conditions is an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine operating conditions by means of an appropriate regression analysis, equation or algorithm.
 9. The method of claim 1, wherein the target CO concentration is predetermined on the basis of a correlation between CO concentration and hydrocarbon (HC) concentration under known engine operating conditions, the target CO concentration being that which corresponds to a predetermined HC concentration at which a predetermined acceptable level of combustion stability is achieved.
 10. The method of claim 1, wherein the target CO concentration is predetermined on the basis of a correlation between CO concentration and hydrocarbon (HC) concentration under known engine operating conditions, said correlation being an approximation of the correlation existing under the operating conditions in use; and wherein the target CO concentration is that which corresponds to a predetermined HC concentration.
 11. The method of claim 1, wherein the target CO concentration is predetermined on the basis of a correlation between CO concentration and hydrocarbon (HC) concentration under known engine operating conditions, the correlation being an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine operating conditions by means of a regression analysis, equation or algorithm; and wherein the target CO concentration is that which corresponds to a predetermined HC concentration.
 12. The method of claim 1, wherein the target CO concentration is predetermined on the basis of a correlation between CO concentration and hydrocarbon (HC) concentration under known engine operating conditions, the target CO concentration being that which corresponds to an HC concentration of approximately 400 ppm.
 13. An engine control unit for a motor vehicle that, in use, performs the method of claim
 1. 14. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 1. 15. A method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is within the predetermined concentration range; and where the calculated concentration of NOx is outside of the predetermined concentration range, adjusting one or more engine and/or combustion parameters so as to achieve an engine-out emission level of NOx within the predetermined concentration range.
 16. The method of claim 15, wherein said one or more engine and/or combustion parameters is EGR.
 17. The method of claim 15, wherein said one or more engine and/or combustion parameters is EGR, and wherein EGR is increased when the calculated concentration of NOx is higher than the predetermined concentration range.
 18. The method of claim 15, wherein said one or more engine and/or combustion parameters is fuel injection timing.
 19. The method of claim 15, wherein said one or more engine and/or combustion parameters is injected fuel quantity.
 20. The method of claim 15, wherein an engine control unit is used to: calculate the concentration of NOx on the basis of the measured concentration of CO; and adjust the one or more engine and/or combustion parameters using a feedback signal.
 21. The method of claim 15, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters.
 22. The method of claim 15, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the means of data comparison is a look-up table, calibration curve or equation.
 23. The method of claim 15, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the correlation is an approximation of the correlation existing under the operating conditions in use.
 24. The method of claim 15, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the correlation is an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine and/or combustion parameters by means of an appropriate regression analysis, equation or algorithm.
 25. An engine control unit for a motor vehicle that, in use, performs the method of claim
 15. 26. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 15. 27. A method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is above a predetermined concentration level; and where the calculated concentration of NOx is above the predetermined concentration level, operating one or more after-treatments to reduce the concentration of NOx in said engine-out emission to a level no greater than said predetermined concentration level.
 28. The method of claim 27, wherein an engine control unit is used to: calculate the difference between the calculated concentration of NOx and the predetermined concentration level; and adjust the one or more after-treatments to reduce the concentration of NOx in said engine-out emission to a level no greater than the predetermined concentration level, and wherein the adjustment to the one or more after-treatments is dependent on the difference between the calculated concentration of NOx and the predetermined concentration level.
 29. The method of claim 27, wherein the one or more after-treatments is selected from the group consisting of: a diesel particulate filter, a catalytic converter, selective catalytic reduction and an NOx trap.
 30. The method of claim 27, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters.
 31. The method of claim 27, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the means of data comparison is a look-up table, calibration curve or equation.
 32. The method of claim 27, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the correlation is an approximation of the correlation existing under the operating conditions in use.
 33. The method of claim 27, wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters, and wherein the correlation is an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine and/or combustion parameters by means of an appropriate regression analysis, equation or algorithm.
 34. An engine control unit for a motor vehicle that, in use, performs the method of claim
 27. 35. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 27. 36. A method for controlling an internal combustion engine to obtain an engine-out emission level of NOx within a predetermined concentration range, the method comprising: measuring the concentration of CO in an engine-out emission; calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO; determining whether the calculated concentration of NOx for the engine under the operating conditions in use is within the predetermined concentration range; and where the calculated concentration of NOx is outside of the predetermined concentration range, adjusting one or more engine and/or combustion parameters so as to achieve an engine-out emission level of NOx within the predetermined concentration range; and operating one or more after-treatments to reduce the concentration of NOx in said engine-out emission.
 37. The method of claim 36, which comprises the steps of: determining whether the calculated concentration of NOx is above a predetermined concentration level; and where the calculated concentration of NOx is above the predetermined concentration level, operating the one or more after-treatments to reduce the concentration of NOx in the engine-out emission to a level no greater than the predetermined concentration level.
 38. An engine control unit for a motor vehicle that, in use, performs the method of claim
 36. 39. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 36. 40. A method for calculating the concentration of NOx in an engine-out emission from an engine, the method comprising: measuring the concentration of CO in said engine-out emission; and calculating the concentration of NOx in said engine-out emission based on a correlation with the measured concentration of CO under the engine operating conditions in use; wherein the correlation is by a means of data comparison, the data relating to measured concentrations of CO and NOx in engine-out emissions under known engine and/or combustion parameters.
 41. The method of claim 40, wherein the correlation is an approximation of the correlation existing under the operating conditions in use.
 42. The method of claim 40, wherein the correlation is an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine and/or combustion parameters by means of an appropriate regression analysis, equation or algorithm.
 43. The method of claim 40, wherein the means of data comparison is a look-up table, calibration curve or equation.
 44. The method of claim 40, wherein the correlation is performed by an engine control unit.
 45. An engine control unit for a motor vehicle that, in use, performs the method of claim
 40. 46. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 40. 47. A method for calculating the engine operating Air-to-Fuel Ratio (AFR) in an engine, the method comprising: measuring the concentration of CO in the engine-out emission from said engine; and calculating the AFR of said engine based on a correlation with the measured concentration of CO under the operating conditions in use; wherein said correlation is by a means of data comparison, said data relating to measured AFR levels and concentrations of CO in engine-out emissions under known engine and/or combustion parameters.
 48. The method of claim 47, wherein the correlation is an approximation of the correlation existing under the operating conditions in use.
 49. The method of claim 47, wherein the correlation is an estimated correlation for the operating conditions in use, the estimated correlation being obtained by adjusting the correlation obtained under the known engine and/or combustion parameters by means of an appropriate regression analysis, equation or algorithm.
 50. The method of claim 47, wherein the means of data comparison is a look-up table, calibration curve or equation.
 51. The method of claim 47, wherein the correlation is performed by an engine control unit.
 52. An engine control unit for a motor vehicle that, in use, performs the method of claim
 47. 53. A motor vehicle comprising an engine control unit that, in use, performs the method of claim
 47. 54. Use of a CO sensor in a system for controlling or adjusting one or more engine and/or combustion parameters in an internal combustion engine, wherein the CO sensor is arranged to measure the concentration of CO in an engine-out emission, and the system includes a means for correlating the measured concentration of CO with one or more of: an indicator of combustion efficiency under the operating conditions in use; the concentration of NOx in the engine-out emission under the operating conditions in use; and/or the level of AFR in the engine under the operating conditions in use.
 55. The use of claim 54, wherein the indicator of combustion efficiency is hydrocarbon (HC) concentration in the engine-out emission.
 56. The use of claim 54, wherein the system controls or adjusts one or more engine and/or combustion parameters in order to bring the calculated concentration of NOx in the engine-out emission to within a predetermined concentration range.
 57. The use of claim 54, wherein the system controls or adjusts one or more engine and/or combustion parameters in order to bring the calculated level of AFR in the engine to within a predetermined range.
 58. The use of claim 54, wherein the indicator of combustion efficiency is hydrocarbon (HC) concentration in the engine-out emission, and wherein the system controls or adjusts one or more engine and/or combustion parameters in order to bring the combustion efficiency in the engine to within a predetermined range. 